TROY, Mich. – A former Oxford Center employee testified that repeated warnings about hyperbaric chamber safety, including static electricity, and children being treated alone, were dismissed by leadership in the years that he worked at the company.
Tamela Peterson, 59, of Brighton, Jeffrey Mosteller, 65, of Clinton Township, Gary Marken, 66, of Spring Arbor and Aleta Moffitt, 61, of Rochester Hills, were charged after Thomas Cooper was killed when the hyperbaric chamber he was receiving treatment in exploded at the Oxford Center in Troy on Jan. 31, 2025. The boy’s mother was also injured in the explosion.
They appeared in court before Oakland County Judge Maureen McGinnis in 52-4 District Court on Monday, Jan. 12, 2026, for a continuation of their preliminary examination.
This was the seventh day the workers appeared in court for the key hearing, during which the judge will determine whether a crime was committed by each defendant and whether enough evidence exists to refer the case to a higher court for trial.
The hearing is expected to resume on Tuesday, Jan. 13, 2026.
Here’s what we learned from the former employee, and two other people who testified on Jan. 12, 2026:
Andrew McMullen, former Oxford Center employee, testifies
Andrew McMullen, a former employee of the Oxford Center (formerly Oxford Recovery Center), testified in court about his years working at the facility, detailing how hyperbaric chambers were operated, how staff were trained, and what he described as repeated safety concerns that were dismissed by leadership.
McMullen worked at the Oxford Center from 2017 until June 2021, beginning at the South Lyon location and later transitioning to the Brighton location when they moved facilities. He started part-time before moving into a full-time role, initially as a hyperbaric technician and later overseeing neurofeedback while co-leading the hyperbaric program. McMullen said he did not work at the Troy facility.
He is a licensed EMT and said that before joining the Oxford Center, he had no prior experience with hyperbaric chambers. All of his training, he testified, happened internally at the Oxford Center.
According to McMullen, safety rules emphasized that patients could only enter chambers wearing approved clothing, typically 100% cotton scrubs provided by the facility, to reduce fire risk. A “safety pause” was conducted to ensure nothing else entered the chamber due to the risk of sparks or explosions.
Once they moved to the Brighton facility, McMullen said patients were allowed to wear pajamas. But, they had to be checked, typically by Peterson or Marken, to ensure the material was suitable for the chambers.
Fire risk and grounding straps
He testified that while grounding in general was discussed, grounding wrist straps were never part of his training. When he later discovered unused grounding straps stored in a laundry room at the South Lyon facility, he asked whether they should be used. He said he was told by Peterson that they were unnecessary.
McMullen testified that he was trained on emergency scenarios including seizures, loss of consciousness, hypoglycemia, and fire. Regarding fires, he said staff were taught that in a monoplace hyperbaric chamber, “there’s not much you can do” for the patient inside.
He told the court he was explicitly taught that a fire inside a chamber would result in total loss and that survival would be “basically impossible” due to the intensity of the explosion. Training focused on containment, shutting off oxygen, emergency venting, and protecting others in the room.
After new Sechrist hyperbaric chambers were installed at the Brighton facility, McMullen said a Sechrist representative discussed grounding and grounding straps during installation. Despite this, grounding straps were still not used.
Safety concerns
McMullen testified that after receiving a painful static shock while cleaning a chamber, he again raised concerns with Peterson. He also described witnessing a child’s hair standing up inside a chamber, which was associated with static buildup based on his training.
He asked whether grounding straps should be used to mitigate static electricity and was again told no. Instead, a humidifier was added to the room.
Later, after speaking with Marken, he obtained a multimeter and began checking the grounding on the chambers himself. He also said Sechrist typically performed annual maintenance checks, though those visits were sometimes delayed due to cost.
McMullen testified that when he first started, children were typically treated in chambers with a parent present. Over time, particularly after the Brighton center expanded and added an Applied Behavioral Analysis (ABA) program for children with autism, children were increasingly treated alone.
He said the shift was driven by higher patient volume. Treating children alone also reduced oxygen usage, as multiple occupants required higher oxygen flow.
McMullen said he raised concerns about diving children alone, describing children as unpredictable and citing incidents involving seizures, equipment being pulled loose, and even fecal matter in chambers. He testified that when he brought these concerns to Peterson, her response was, “Deal with it.”
According to McMullen, Peterson was the ultimate authority at the Brighton facility and determined all treatment plans. Even nurses, he said, sought her final approval. Marken would also escalate concerns to Peterson if he could not resolve them.
Dive logs and operating chambers
The facility had cameras set up in the chamber rooms and throughout the center. McMullen testified that Peterson had camera feeds from both the Brighton and Troy facilities displayed in her office and could closely observe chamber operations.
McMullen described the transition from handwritten dive logs to electronic charting after the move to Brighton. Initially, staff were told to prefill logs, but later were instructed by Peterson to document times accurately as the center sought insurance coverage for specific conditions.
On busy days, McMullen said all five chambers at the Brighton facility could be in use for six or more hours, sometimes with a single technician responsible for operating all of them during opening or closing shifts.
Dive counts
In testimony that drew objections from the defense, McMullen described witnessing Gary Marken manipulating dive counters on older chambers using a screwdriver. McMullen said Marken told him the counters were “overcounting” and needed to be rolled back to what they believed was accurate.
McMullen puts in his two-week notice
McMullen left the Oxford Center in June 2021, citing frustration with daily operations and ongoing safety concerns that he felt were dismissed. He also said he was preparing to start a family and relocate.
He testified that when he gave his two-week notice, Peterson reacted angrily and told him he should have given more notice and would “regret” leaving. He described a tense and unpleasant conversation with both Peterson and Marken after he put in his two weeks.
Cross-examination
During cross-examination, former Oxford Center employee Andrew McMullen acknowledged that his knowledge of the facility is limited to the period when he worked there, from 2017 to June 2021, and that he has no personal knowledge of operations, safety culture, or management decisions made after he left.
McMullen confirmed he has followed media coverage of the case and said he contacted police and someone from the attorney general’s office after he learned about the explosion. He met with prosecutors after receiving a subpoena, including a Zoom meeting with members of the attorney general’s office, and ultimately signed a confidential informant agreement following the fatal chamber fire.
Defense attorneys emphasized that McMullen received mandatory on-the-job training and job shadowing when he was hired. He testified that early training at the South Lyon location was informal and conducted day-to-day, without written materials. More formal instruction occurred later, after the move to the Brighton facility, including a 40-hour course with textbooks, PowerPoint presentations, testing, and certification.
He confirmed that Jeffrey Mosteller served as the center’s safety director and led much of the formal training, along with other staff.
A major focus of cross-examination involved patient grounding wrist straps. McMullen repeatedly testified that grounding straps were not used at the Oxford Center during his tenure and that he was told by leadership they were unnecessary.
He acknowledged that Sechrist, the chamber manufacturer, installed grounding straps with the chambers and mentioned their absence during maintenance visits. When questioned about this, McMullen said Sechrist employees made comments that operating without the straps was “not a good idea,” but never instructed staff to stop using the chambers.
McMullen testified that he personally underwent hyperbaric treatment after a motorcycle accident and did so without wearing a grounding strap, as he had been told it was not required. He also confirmed that Peterson, her husband, her daughter, her father, and other family members frequently used the chambers without grounding straps.
McMullen agreed that Gerald Gleeson Peterson was heavily involved in the Oxford Center’s Applied Behavioral Analysis (ABA) program for children with autism. He testified that many child patients were nonverbal and that the facility used extensive video monitoring, though he said he did not know whether cameras were installed primarily for that reason.
He confirmed that decisions about whether children could dive alone were made by Peterson, not by technicians.
McMullen testified that he and another employee were the only full-time hyperbaric technicians at the Brighton facility and were responsible for scheduling chamber times, but not for determining staffing levels. He said decisions about bringing in additional part-time technicians were made by Marken.
Throughout the cross-examination, McMullen agreed that he cannot speak to the Oxford Center’s culture, management structure, or safety practices from 2022 onward. He also acknowledged that while he understood the power structure at the facility, he did not know the formal titles or positions held by leadership in later years.
He said decisions ultimately rested with Peterson, and that while he believed Mosteller provided more structured training later on, he did not have the authority to implement changes McMullen felt were needed.
During the prosecution’s redirect, McMullen testified that his concerns about children being treated alone in hyperbaric chambers stemmed from their unpredictability, describing incidents of children roughhousing, dumping water where the oxygen comes in, and tampering with equipment. He said his primary fear was not damage to the chambers but the risk of children hurting themselves and the delay in reaching them during an emergency, noting that movement can create friction and static, and that at least one patient reported a painful static discharge during treatment.
When a defense attorney asked McMullen if he reported this to anyone, he said, “I talked to Tammy. It was a personal matter.”
McMullen also told the court that an article about static discharge in chambers and that grounding straps weren’t necessary was unconvincing because it assumed ideal conditions and did not account for real-world variables like lint or frayed bedding.
While Peterson appeared to care about patients, he testified that when he raised safety concerns, they were “mainly brushed off.”
In addition to McMullen, two other witnesses testified.
Troy Det. Ryan Whiteside testifies about chart that tracks boy’s treatments before explosion
On Jan. 12, 2026, Troy Detective Ryan Whiteside was recalled after previously testifying during the hearing.
He was asked about a chart he created that documented Thomas Cooper’s treatments.
Whiteside reviewed video from 35 hyperbaric chamber treatments Thomas received beginning Dec. 9, 2024, and created a chart to track how long treatments lasted, how Thomas entered and exited the chamber, and whether safety checks appeared to have happened.
Whiteside testified that Thomas showed some level of movement during every recorded treatment, sometimes more active and other times watching TV, and was always accompanied by his mother in the chamber treatment room.
Whiteside said he expected some form of pat-down or hands-on inspection during the safety check, but didn’t see that. He was not told that a physical pat-down was not required. He also acknowledged he could not hear conversations between staff and Thomas’ mother, so he couldn’t hear what was being talked about during the safety checks.
He also testified that he did not document Marken in any of his reports.
Troy Det. Troy Police Detective Brenna Yunkari testifies about phone call during investigation
Det. Brenna Yunkari testified that she assisted with the investigation beginning the day of the explosion, helping follow up on leads and return calls as directed by the lead detective.
On Feb. 1, 2025, she had a phone call with a man who contacted police, claiming he had decades of hyperbaric experience, military and hospital training, and early certification in the field.
The man offered opinions that the Oxford Center’s chambers were well maintained and that grounding wrist straps were “secondary” and “precautionary,” according to the defense.
Yunkari said she conducted basic online research during the call, found limited publicly available information about him online, and ultimately flagged concerns to her supervisors that the man’s assessment appeared potentially biased, noting his repeated praise of the Oxford Center and desire to access the scene.
She testified that police declined his offer of assistance, did not open the scene to him, and that decisions about experts and further follow-up were made by the supervisor and the lead detective, not her.
What’s next
Yunkari’s was the last witness to testify before court concluded for the day.
The hearing is scheduled to continue on Tuesday, Jan. 13, 2025. The preliminary examination cannot be livestreamed, per the judge, but we will continue to have coverage for you on ClickOnDetroit.com.
For a full deep dive into what we learned during previous days of the preliminary hearing, check out the following stories: