The Nigerian suspected of trying to bring down a U.S. airliner with explosives might have used Arabic studies as a pretext for entering Yemen before disappearing for months, perhaps into one of the lawless country's al-Qaida strongholds, fellow students and teachers said.

Interviews with staff and students at the Institute for the Arabic Language this week have revealed that Umar Farouk Abdulmutallab was frequently absent from classes and spent at most one month at the school in Yemen's capital, San'a, starting in late August before vanishing. Authorities say he didn't leave the country until December and that his whereabouts during that period remains a mystery.

Adding to the notion that his studies were a ruse, one teacher at the institute said the young man stood out because he already spoke Arabic with a considerable degree of fluency.

"I noticed he understood the language very quickly and he was eloquent and convincing in his speech," Ahmed Moajjib said.

U.S. investigators have said the 23-year-old Abdulmutallab told them he received training and instructions from al-Qaida operatives in Yemen. He is accused of trying to detonate an explosive device hidden on his body as a Northwest Airlines flight carrying 289 people approached Detroit on a flight from Amsterdam on Christmas Day.

His links to Yemen have focused attention on al-Qaida's growing presence in the impoverished and lawless country on the tip of the Arabian Peninsula. Al-Qaida in the Arabian Peninsula, an offshoot of Osama bin Laden's group, claimed it was behind the attempt to bomb the flight.

Yemen's government has said Abdulmutallab spent two periods in the country, from 2004-2005 and from August to December of this year. He was enrolled at the language school during both periods to study Arabic.

Information Minister Hassan al-Lozy said on Tuesday that investigators were working on the assumption that Abdulmutallab entered the country "under the pretext of studying Arabic during his last stay."

Another teacher at the institute, Ahmed Mohammed, said Abdulmutallab spent the last 10 days of the holy month of Ramadan sequestered in a mosque and attended barely four hours of the 20-hour course he enrolled in.

On Thursday, Yemeni security officials said Abdulmutallab overstayed his visa by three months and should have been stopped by authorities from leaving the country in December. The visa was valid from Aug. 4 to Sept. 4. The language institute said he received a visa extension until Sept. 21.

Ahmed Hassan, a 28-year-old Arabic language student from Singapore who was his roommate at the institute, said Abdulmutallab spoke of wanting to also study in Yemen's eastern Hadramawt province.