PITTSFIELD TOWNSHIP, Mich. -- A high school that was forced to evacuate twice this week when more than a dozen students mysteriously became ill is closed Friday.

Extra-curricular activities scheduled at other locations will take place Friday, but no events will be held at Saline High School, Local 4 reported.

Crews ran more tests at the high school overnight in an ongoing investigation to determine the cause of the students' illness. The illness has been traced to two classrooms located on different floors and opposite sides of the building, Local 4 reported.

Four students went to the hospital Thursday, with a total of 15 students who have become ill at the school this week, Local 4 reported.

Student Rebecca Calvitt said that one of her friends who became sick "was utterly confused as to what was going on."

The students who became ill Thursday had similar symptoms of nausea as students from the previous two days, according to Betty Rosen-Leacher, executive director of school instruction. Rosen-Leacher said the Pittsfield Township Fire Department was called in to investigate Thursday, and asked school officials to evacuate the building again.

About 1,800 students and faculty members were evacuated and directed to the school's football field. At the time of Thursday's evacuation, environmental crews were inside the building testing the air to determine the source of the illness.

"None of those instruments were detecting anything in the air," said Pittsfield Township Fire Department Chief Al D'Agostino.

The high school was also evacuated on Tuesday after five students passed out and reported symptoms of nausea, according to Local 4 reports. Classes resumed at the school on Wednesday, but six more students became ill.

The school closed a classroom Wednesday while environmental consultants continued to search for the source of the problem, Local 4 reported.

Initial environmental test results on Tuesday showed no unusual toxins in the building, according to Local 4 reports. A second test conducted by an independent company was also negative for toxins, the station reported.

No information was available as to whether classes would resume Monday.