Umar Farouk Abdulmutallab, 23, stayed in Yemen for a year between 2004 to 2005, before returning for another stay from August 4 to September 21 of last year, Yemeni officials have said.
A teacher at the Sana Institute for the Arabic Language, who taught Abdulmutallab during his time in Yemen, said he was closer to being secular when he first arrived in the country.
But during his visit last year, he was more committed to praying and Islam, the teacher said.
A fellow student who holds U.S. citizenship said Abdulmutallab had not exhibited any violent tendencies and a community leader in the neighbourhood where the institute is located said he had been very friendly to cleaning staff, even offering them chocolates.
Nobody expected he would harm anyone, the student said.
Abdulmutallab was taken into custody after being overpowered by passengers and crew as the U.S. passenger jet approached Detroit on Christmas Day after taking off from Amsterdam.
Yemeni authorities have tightened entry-visa regulations for applicants looking to join Arabic language institutes, after the December 25 incident, a Yemeni security official said.
Foreigners of different nationalities have been known to travel to Yemen's language institutes, mainly in the capital Sanaa, to study Arabic.The institute where Abdulmutallab studied teaches mostly American, Canadian and Malaysian students, who are either Muslims, students of Middle Eastern studies or researchers, the teacher said.
Authorities suspect Abdulmutallab moved to al Qaeda's main hideout in Yemen sometime between September of last year and December 7, when he boarded a flight to Ethiopia, the security official said.
The official said Abdulmutallab's visa allowing him to stay in Yemen had expired when he left the country last month and that it was unclear why this had not prompted airport security to examine his case more closely.
The Nigerian youth, who had been studying for his masters degree in Dubai before returning to Yemen in August, comes from a wealthy family and is the son of a prominent former banker.
Nigerian officials said the suspect has been living outside Nigeria for a while and snuck back into the country on December 24, before departing the same day.