In more than a dozen interviews Wednesday, those who know Umar Farouk Abdulmutallab shared impressions of the man who studied in Yemen last summer only to vanish in October, and emerge on Christmas Day on a Detroit-bound flight from Amsterdam with chemical explosives allegedly sewn into his underwear.

SAN’A, Yemen —

The young Nigerian man had visited Yemen once, in 2005. By the time Umar Farouk Abdulmutallab returned last August, again to study Arabic, he appeared to be a different person — more deeply religious, more of a loner, and forsaking Western clothing in favor of a long, white traditional Islamic tunic.

Abdulmutallab, 23, also expressed an inner confidence and a certainty of purpose, according to former teachers, classmates and housemates. He seemed to be on a mission, spending long hours in a mosque, often missing classes, and even ordering a classmate to stop smoking in front of him.

In more than a dozen interviews Wednesday, those who know him shared impressions of the man who studied in Yemen last summer only to vanish in October, and emerge on Christmas Day on a Detroit-bound flight from Amsterdam with chemical explosives allegedly sewn into his underwear.

Teachers said Abdulmutallab bears little resemblance to the man they remembered.

“He was so open-minded in 2005. He spoke to everybody,” said an instructor at the San’a Institute for the Arabic Language. The teacher spoke on the condition of anonymity because Yemeni security officials had ordered staff not to talk to journalists. “In 2009, he barely came to class. I wouldn’t see him in a week.”

When Abdulmutallab spoke, he was courteous. He didn’t publicly express radical thoughts, didn’t lash out against U.S. policies in Iraq or Afghanistan. He didn’t express core Muslim grievances such as Israel’s treatment of Palestinians. He often handed out money to the poor.

“He was friendly. He always smiled. We didn’t see an ounce of aggressive behavior,” said Ahmed Mujaeb, a teacher. “I felt sad when I heard what he had done. I asked myself ‘Why?’ This is a big question mark.”

Yemeni investigators are trying to piece together what happened to Abdulmutallab between October and December, when he apparently left Yemen. He allegedly told U.S. authorities he was equipped and trained by a bomb-maker linked to al-Qaida in the Arabian Peninsula, which has claimed responsibility for the Northwest Flight 253 plot.

An unwanted spotlight now shines on the institute, housed in a tan, three-story mansion with a tree-speckled courtyard in the historic quarter of San’a. Since 2001, the school has taught Arabic to hundreds of foreign students, including many Americans. Yet, students and teachers said they feared the school would be unfairly tainted as a jihadist breeding center.

Under the online name Farouk1986, Abdulmutallab wrote in June 2005 that the school was “great.” He gushed about Britons and Americans in the capital and about eating at Pizza Hut and KFC.

When he returned this time, he was not seen eating fast food, said Ahmed Hassan, a classmate from Singapore who lived next door to Abdulmutallab in the school’s residence building. Every morning, he woke up before dawn to pray. He ate breakfast in his room or alone in the cafeteria, his housemates said. He spoke Arabic well and had improved tremendously since 2005, when he was enrolled in a beginner’s class, his instructors said.

Abdulmutallab did not listen to music; the only sound heard from his room were his recitations of the Quran. One day, the call for prayer from a mosque floated into the classroom. Abdulmutallab stood up and said he must pray in the mosque, and left, Hassan recalled.

In September, during the last 10 days of the Muslim holy month of Ramadan, his teachers said Abdulmutallab performed itikaf, a spiritual retreat in which devout Muslims spend evenings and nights in a mosque, worshipping and reading.

On the few instances he had conversations with classmates, the discussion inevitably turned to religion. “He believed in the infallibility of the Quran,” said Matthew Salmon, a Canadian student.

Abdulmutallab spoke English with a West African clip, and frequently mixed conversations with Quranic terms. He didn’t discuss family or personal matters, Hassan said. Most students thought he was poor; they were surprised to learn his family is wealthy.

When inside his room, he often locked the door. Hassan said he peeked inside one day. There were no family photos and only a laptop on his desk. The walls were bare.

By the end of September, Abdulmutallab seemed intent on learning Islamic sharia law. Hassan and other classmates said he wanted to attend an Islamic school in Hadhramaut province, where al-Qaida has deep roots. He told other classmates that he wanted to be an engineer.

Salmon said he asked Abdulmutallab how long he planned to stay. He said he would stay another month or two, then return to London, where he attended college.

Abdulmutallab disappeared the next day. “He didn’t even say goodbye to us,” Hassan said.

Some classmates now wonder if Abdulmutallab’s decision to return to the school was merely a pretext, a way to gain entry to Yemen to conduct a terrorist act.

“My roommates and I are very frustrated,” Salmon said. “This damages the credibility of the school, of Yemen, and of Islam itself.”