The young Nigerian man had visited Yemen once before, in 2005. But by the time Umar Farouk Abdulmutallab returned this past August, again to study Arabic, he appeared to have become a very different person, more deeply religious, more of a loner, and forsaking Western clothing in favor of a long, white traditional Islamic tunic. Abdulmutallab also expressed an inner confidence and a certainty of purpose, according to former teachers, classmates and housemates. The 23-year-old 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 on Wednesday, those who know him shared impressions of the man who joined them for language school in Yemen this 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. At a dinner in September, he demanded that classmate Sigurd Sorensen walk behind him as Abdulmutallab said his evening prayers. 'What faith are you?' Abdulmutallab demanded. 'Christian,' Sorensen replied. 'Islam is the only true way,' Abdulmutallab then declared, as Sorensen recalled. 'If you want to go to heaven, you should accept it.' Teachers said Abdulmutallab bears little resemblance to the man they remembered from four years earlier. 'He was so open-minded in 2005. He spoke to everybody,' said one of his instructors at the San'a Institute for the Arabic Language. The teacher spoke on the condition of anonymity because Yemeni security officials had ordered the school's 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 poor Yemenis and African migrants. '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 that he was equipped and trained by a bomb maker linked to al-Qaida in the Arabian Peninsula, which has claimed responsibility for hatching the plot to bomb Northwest Flight 253. It has shined an unwanted spotlight on the institute, housed in a tan three-story mansion with a tree-speckled courtyard in the historic quarter of San'a. Since 2001, it has taught Arabic to hundreds of foreign students, including many Americans. It offers one religious course, a basic introduction to Islam to help students learn classical Arabic. 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 a June 2005 posting 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 his housemates heard from his room were his recitations of the Koran. One day, the call for prayer from a mosque floated into the classroom. Abdulmutallab stood up and informed the class that he must go and 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 where devout Muslims spend evenings and nights in a mosque, worshiping and reading the Koran. The few instances he had conversations with classmates, the discussion inevitably turned to religion. 'He believed in the infallibility of the Koran,' said Matthew Salmon, a Canadian student. Abdulmutallab spoke English with a West African clip, and frequently mixed his conversations with Koranic terms. He didn't discuss his family or personal matters, Hassan said. Most students thought he was poor; they were surprised to learn recently that his family is wealthy. His room was spare; when he was inside, he often locked the door. One day, Hassan managed to peek inside, he said. There were no photos of Abdulmutallab's family. The only item on his desk was a laptop. The walls were bare. By the end of September, Abdulmutallab seemed intent on learning Islamic sharia law. He told Hassan and other classmates that he wanted to attend an Islamic school in Hadhramaut, a southern province where al-Qaida has deep roots. He told other classmates that he wanted to be an engineer. One night in late September, Salmon said he asked Abdulmutallab how long he was planning to stay at the school. He told Salmon that he would stay another month or two, and then return to London, where he attended college. The next day, Abdulmutallab disappeared. 'He didn't even say goodbye to us,' Hassan said. Questions and anger now swirl around the school. Some classmates wonder if his decision to return to the school was merely a pretext, a way to gain entry to Yemen to conduct a terrorist act. Others made it a point to show a visiting journalist one of the school's textbooks -- to prove there were no references to jihad or terrorism. 'My roommates and I are very frustrated,' Salmon said. 'This damages the credibility of the school, of Yemen, and of Islam itself.' 