DETROIT -

His suicide mission on a plane headed for Detroit on Christmas Day 2009 could have ruined the lives of hundreds. But the bomb inside Umar Farouk Abdulmutallab underwear failed to go off, and hygiene appears to be the reason why.

The Nigerian man was trained in Yemen under Anwar al-Awlaki, a radical American-born cleric and one of the best-known al-Qaida figures. Abdulmutallab wore the explosive in his underwear aboard Northwest Airlines Flight 253, which originated in Amsterdam with nearly 300 people aboard. The bomb didn’t detonate, but did start a small fire in his lap.

Now, the head of the Transportation Security Administration John Pistole said the bomb never went off because of how long Abdulmutallab had been wearing it.

“The bomber had the device with him for over two weeks,” Pistole said.

When asked if the explosive had become damp, Pistole said, “Let’s just say it was degraded.”

--A sketch of Abdulmutallab during his trial

Abdulmutallab pleaded guilty and was sentenced to life in prison without parole in 2012. He’s at a maximum security federal prison in Fremont County, Colorado.