By SARAH CHILDRESS And WILL CONNORS
KADUNA, Nigeria—As a child in this northern Nigerian town, Umar Farouk Abdulmutallab used to chastise his banker father for not giving more money to the poor, invoking his family’s adherence to the tenets of Islam.
“He preached to his father all the time,” said Mahfuz Datti, Mr. Abdulmutallab’s childhood friend.
This week brought word of a different sort of family gathering. Mr. Abdulmutallab, who allegedly tried to blow up a Detroit-bound airliner on Christmas Day with explosives sewn into his underwear, began cooperating with federal law-enforcement agents last week, U.S. officials said Tuesday. Providing rare behind-the-scenes detail of how the U.S. is handling the case, these officials said Mr. Abdulmutallab ended a month of silence after receiving visits over several days from family members.
The terrorism allegations against Mr. Abdulmutallab have brought international attention to his father, Alhaji Umaru Mutallab, one of Nigeria’s richest men, who approached U.S. authorities in November with concerns about his son’s radicalization. The latest family visit appeared to mark a stark split between the two: On Wednesday, officials confirmed that Mr. Abdulmtallab’s mother and siblings, not his father, were at the visits. “The father and son’s relationship is broken,” said a senior U.S. law-enforcement official.
An examination of the lives of the 70-year-old father and the 23-year-old son shows they were shaped by similar experiences and shared many traits, including a withdrawn seriousness and devotion to Islam. The father became one of Nigeria’s top bankers, with extensive Western and Nigerian contacts. The son, who came of age amid sporadic religious clashes in his hometown and attended elite schools abroad, would ultimately be drawn to violent extremists.
Mr. Mutallab, through an assistant, declined requests for an interview, as did an attorney for Mr. Abdulmutallab, who has pleaded innocent to charges against him. Family members declined to respond to questions about their background. Officials from the State Department and Federal Bureau of Investigations declined to elaborate further on the family’s role in the ongoing investigation.
Accounts of father and son are drawn from interviews with friends and associates in their town of Kaduna, about two hours north of Nigeria’s capital, Abuja.
Mr. Mutallab, the son of a construction magnate, was born into a life of privilege. Surrounded by wealth in a crowded and poor country, he left home at a young age to attend British-style boarding schools, including Barewa College preparatory school in Zaria, Nigeria, whose alumni include business leaders and Nigeria’s current president. Friends say he was a driven, above-average student.
Mr. Mutallab studied accounting in the UK and returned to Nigeria in the mid-1960s, working in government-run defense and development companies. He later established a dairy farm and Nigeria’s first bottling plant for Dr. Pepper cola, and in 2003, helped found Jaiz International PLC, Nigeria’s first major Islamic bank.
Mr. Mutallab typically rises early to attend prayers, those who know him in Kaduna said. He is known as Alhaji, someone who has made Hajj, the spiritual journey to Mecca.
Friends say his career was unconventional only for its lack of controversy. “He’s one of the few public figures who there’s never been a scandal about,” said Jibrin Ibrahim, director of the Center for Democracy and Development in Abuja. “Not even a story.”
As his wealth grew—he owned spacious homes in Kaduna, Lagos and Abuja—he drew criticism from his son for not giving enough to others. The sparring seemed to please the equally devout father, friends of both say.
The son, Mr. Abdulmutallab, grew up primarily here in Kaduna, a onetime British colonial administrative center, in a large, pale yellow house behind a high wall lined with palm trees. When he was three or four years old, he attended his first lessons in Islam at a nearby madrassa funded in large part by his father. A teacher there recalled an eager and unfailingly obedient boy who soaked up knowledge.
He left Kaduna to attend a series of elite schools, returning home for holidays. In many ways, he was an ordinary teen, acquaintances say: He eschewed the Muslim cap and caftan for jeans and T-shirts, and sometimes stood outside his front gate in a track suit, headphones on, arms crossed. He had a boxed set of the TV show “Friends.” He rode a red-and-blue motorbike—sometimes too fast, neighbors say, a rare break from his obedient character.
He had few if any friends, say those who knew him. One was Mr. Datti, who played with Mr. Abdulmutallab when they were children and recalled his conversations with his father. Mr. Datti, who agreed to speak through a common acquaintance, said he was devastated by the arrest. He says the two friends spent their younger years together praying and talking about their faith. “Whatever he was doing, he wouldn’t miss prayer time,” Mr. Datti said.
Other young men called him ustaz, or religious man.
As Mr. Abdulmutallab entered his teen years, change swept Kaduna, which sits on the intersection of Nigeria’s Muslim and Christian areas.
For the most part, members of both faiths lived together peacefully. But in 2000, Kaduna state’s governor decided to rule in accordance with Islam’s strict Shariah law. Christians revolted. About 1,000 people were killed in the ensuing violence. Since then, many residents say, the Muslim community has become more radicalized.
Mr. Abdulmutallab’s education took him to Togo and Yemen. Starting in 2005, when he began studying in London, he began flirting with more radical forms of Islam, according to U.K. intelligence reports.
Returning to Kaduna in 2008, he appeared more reticent than before, his friend said. “He seemed always deep in thought,” said Mr. Datti.
Mr. Abdulmutallab appeared increasingly intolerant of those he felt weren’t being true Muslims. At one prayer session that year, Mr. Datti’s cell phone rang in the mosque, its ringtone from a song by American rapper Tupac Shakur. “Why are you acting like you’re not a Muslim?” Mr. Abdulbutallab said, snatching the phone, according to Mr. Datti.
Mr. Abdulmutallab was also concerned about the suffering of Muslims in Palestine, Iraq and Afghanistan. He had rarely watched television or read the newspaper, Mr. Datti said. But now he followed these events closely, and spoke about them with Mr. Datti after prayers at the mosque, sometimes biting his lower lip. “He would say, ‘Why, why, why!'” Mr. Datti said.
Mr. Datti said his friend never spoke of committing violence, but believed Muslims of all sects should be united to better defend the faith.
His father, Mr. Mutallab, became worried the next year, when his son interrupted his studies towards a master’s degree in Dubai to travel to Yemen. He approached Nigerian security agencies and the U.S. Embassy, but not to report any possible terrorist attacks, friends say. The son had indicated via text message that he was going to cut off all contact with his family.
“He thought the Americans could be of assistance, because they have links around the world,” said Lema Jibrin, a family friend. “He just wanted to find out where [his son] was so they could talk with the child and bring him home. Nobody thought he could join al-Qaeda or anything like that, not at all.”
Mr. Datti said he fell out of touch with his friend during that time, losing his cell phone and with it Mr. Abdulmutallab’s phone number.
Mr. Datti says he was surprised when, last Dec. 23, two days before Christmas, he saw his friend drive past in a green Honda with a Lagos license plate. Mr. Datti says he shouted and waved but that Mr. Abdulmutallab honked the horn and kept driving.
His account couldn’t be corroborated. The Nigerian government says Mr. Abdulmutallab flew into Lagos on Dec. 24 before continuing toward the Amsterdam-Detroit flight he allegedly attempted to bring down.
“I waited for him to come back until the call to evening prayer,” said Mr. Datti. When Mr. Abdulmutallab didn’t return, he went to the mosque, expecting to see him. He wasn’t there.
—Evan Perez in Washington, D.C., contributed to this article.
Write to Sarah.Childress@wsj.com