The notorious underwear bomber's explosives failed to go off during the 2009 Christmas Day plot because he wore the potentially deadly undies for three weeks before trying to detonate them, FBI agents involved in the investigation said.

Two FBI agents told ABC News' Detroit affiliate WXYZ in an exclusive interview that when the bomber, Umar Farouk Abdulmutallab, confessed to the plot, he went on to say that he had worn the explosive underwear for three weeks in order to get used to it and make sure he could get it through security.

"We think ultimately, that probably is what caused a little bit of separation in the sequence of events in the explosion," said FBI agent Ted Peissig, who interrogated Abdulmutallab.

Abdulmutallab pleaded guilty last year to attempting to blow up Northwest Flight 253 over Detroit on Christmas Day in 2009. He was able to get through security and onto the plane, but the underwear bomb he was wearing failed to detonate. Instead, it caught fire, severely burning Abdulmutallab.

The young jihadist was given multiple life sentences, but not before saying in court that he was still " proud to kill in the name of God."

The agents also revealed to WXYZ that Abdulmutallab was not recruited by al Qaeda, but the other way around.

"He sought out al Qaeda relentlessly," another FBI agent on the case, Mike Connelly said. "He persisted and he was almost turned away at times, you know, by al Qaeda. But he refused to relent."

Connelly was one of the FBI agents tapped with traveling to Abdulmutallab's hometown in Nigeria shortly after the incident where WXYZ said he "uncovered critical intelligence in the war on terror, and gave agents a rare glimpse inside the mind of a suicide bomber."

CLICK HERE to see WXYZ's 3-Part Series on the untold story of the Underwear Bomber.