(CNN) -- Investigators combed through Umar Farouk Abdulmutallab's history Sunday in hopes of learning how the British-educated son of a Nigerian bank executive ended up carrying what authorities said was a bomb onto a Michigan-bound jetliner.
Abdulmutallab was released from a hospital in Ann Arbor, Michigan, on Sunday after being treated for burns, according to Gina Balaya, a spokeswoman for the U.S. attorney's office in Detroit, Michigan. The 23-year-old is charged with attempting to set off an explosive device aboard a Northwest Airlines flight from the Netherlands shortly before its landing in Detroit on Christmas Day, and was being held in an undisclosed location, Balaya told CNN.
Meanwhile, investigators were trying to retrace Abdulmutallab's steps halfway across the globe, from the Arabian peninsula to West Africa to Britain.
Abdulmutallab claimed to have extremist ties and said the explosive device -- made from a plastic explosive known as PETN -- "was acquired in Yemen along with instructions as to when it should be used," according to a federal security bulletin.
A source with knowledge of the investigation told CNN Sunday that the amount of explosive was sufficient to blow a hole in the side of the aircraft.
U.S. intelligence warns that the al Qaeda terrorist network has established a foothold in Yemen, the ancestral homeland of its founder, Osama bin Laden, where government troops are now battling both al Qaeda operatives and a Shiite Muslim separatist movement.
In Britain, where the suspect studied engineering at a London university, police spent a second day searching his last known apartment. Bombings by home-grown Muslim radicals killed 52 people in London in 2005.
"This man had been in London, where there is frequent evidence of recruitment by al Qaeda, al Qaeda-related people," said John McLaughlin, the former deputy director of the CIA, now a CNN analyst. "He claims to have been in touch with Yemenis, and Yemen is a place where al Qaeda is on the move."
Detectives with Scotland Yard's anti-terrorism branch spent part of Sunday interviewing Michael Rimmer, a former high school teacher who described Abdulmutallab as a "very devout" Muslim who had once expressed sympathy for Afghanistan's Taliban insurgency during a classroom discussion. But Rimmer, who taught Abdulmutallab at a school in the West African nation of Togo, said it was not clear whether the then-teenager was simply playing devil's advocate during the class.
The site of the London search was a basement apartment in an ornate building in a wealthy neighborhood. Abdulmutallab returned to London in May, and received a tourist visa from the U.S. Embassy there that he used to fly to Detroit from the Nigerian commercial capital of Lagos via Amsterdam, the Netherlands.
A family source said Abdulmutallab received a college degree at University College London, where spokesman Dave Weston said a man named Umar Farouk Abdul Mutallab was enrolled in the Mechanical Engineering Department between September 2005 and June 2008.
He then went to Dubai, the UAE's financial hub, around January 2009 to study for a masters degree in international business.
A U.S. official said Saturday that Abdulmutallab's father contacted the U.S. Embassy in Nigeria several weeks ago to report that his son had "become radicalized." The father, Umaru Abdulmutallab, told the embassy the family feared his son had gone to Yemen to participate in "some kind of jihad," the family source said.
The source told CNN that Abdulmutallab's father -- who recently retired as chairman of First Bank PLC, one of Nigeria's premier banks -- had his concerns raised by a text message from his son. But a senior Obama administration official with knowledge of the case said the father's call amounted to little more than a "missing person report."
The official, who could not speak on the record because of the sensitive nature of the information, said the elder Abdulmutallab was deemed credible as a source, but did not have enough specific information to justify canceling his son's U.S. visa or putting him on a "no-fly" list.
"If we pulled his visa or banned him from flying, that would have alerted him you are onto him," the official said. "Whereas in some cases if you have a terror lead, you watch to see what happens when he travels, which could be more valuable." In addition, the official said, it was "fairly early in the process of what kind of threat this guy poses."
The father's information was vetted and a determination on how to proceed was made through a U.S. inter-agency process, and a file was opened on Abdulmutallab about five weeks ago. But the official added, "One part of the system that absolutely failed" was that Abdulmutallab was able to board a plane to the United States allegedly with PETN.
Abdulmutallab first boarded a KLM flight from Lagos, Nigeria, to Amsterdam. Both Nigerian authorities and KLM, a Northwest partner, say he received secondary security screening. But a U.S. official said Friday that Abdulmutallab was not checked for traces of explosives using a more extensive test.
CNN's Jason Morris, Elise Labott and Paul Cruickshank contributed to this report.