The FBI has secretly arrested a Maryland juvenile who allegedly conspired in a terrorism plot with the Philadelphia-area woman known as Jihad Jane, sources have told The Inquirer.

The boy is 17 but was 15 when he conspired with Colleen LaRose of Pennsburg, Montgomery County, to solicit money and recruits for a jihad, according to documents and sources. His case is sealed in U.S. District Court in Philadephia.

His family emigrated from Pakistan four years ago, and relatives say the boy - Mohammed K., of Ellicott City, Md. - was headed to Johns Hopkins University on a full scholarship this fall. They also say he was questioned by the FBI, without a parent or lawyer present, at least eight times.

"Now we know that was a mistake," a relative said. "We had thought everything was taken care of and fine because he talked to the FBI so many times - but the next thing you know, a year later, without any warning, the FBI took Mohammed away. It was a shock to us and to him."

Family members spoke on condition of anonymity. The Inquirer is not publishing the boy's last name because he is a juvenile and the specific charges have not been made public.

Federal charges against juveniles are rare. Nationally, only 100 juveniles are serving federal sentences, and federal officials could not cite another juvenile who has been arrested on terror-related charges.

Mohammed's arrest came six months after LaRose, who called herself "Jihad Jane" on websites, pleaded guilty to conspiracy to provide material support to terrorists, including providing a U.S. passport, and lying to FBI agents about it. A coconspirator, Jamie Paulin-Ramirez of Leadville, Colo., pleaded guilty to providing material aid to terrorists.

U.S. counterterrorism officials have said the Jihad Jane case represents a new and alarming threat - suburban, American-born women aiding Islamic terror groups.

Prosecutors said LaRose worked obsessively on her computer "to communicate with, recruit, and incite other jihadists." In 2009, she agreed to help try to kill the Swedish artist Lars Vilk, whose 2007 drawing of a dog with the head of the prophet Muhammad offended some Muslims.

Mohammed K.'s relatives said they do not know much about the allegations - or what Mohammed told the FBI - but are confused and angry that they allowed the boy to spend so much time with agents.

"When they said, 'Can we take him out for a few hours?' it seemed so informal," one relative said. "And now, in a way, we feel cheated."

FBI spokesman J.J. Klaver and other federal officials declined to comment.

Mohammed's court-appointed lawyer, Jeffrey Lindy of Philadelphia, said, "This is a juvenile matter and is confidential, so it would be inappropriate to comment." Mark Wilson, a federal public defender representing LaRose, also declined to comment.

LaRose traveled to Ireland in September 2009 to meet several coconspirators. She offered to use her U.S. identity and her boyfriend's passport, and to marry a jihadist to help with the terror plot, officials said. The plot fizzled for reasons that have not been made public.

The FBI believes that Mohammed was part of the conspiracy, and that he met LaRose in a jihadist chatroom, sources said.

The only people publicly charged in the United States are LaRose and Ramirez. The other alleged coconspirators - including Mohammed and the Irish suspects - are cited in the LaRose indictment only by geographic location, numbers, and "CC," the code for coconspirator. In the public document, sources said Mohammed is "CC#4, a resident of the United States."

The indictment alleges that in July 2009, when Mohammed was 15, he posted "an online solicitation for funds to support terrorism on behalf of defendant Colleen R. LaRose, a.k.a. Fatima LaRose, a.k.a. Jihad Jane."

"I write this message on behalf of a respected sister," Mohammed allegedly wrote. "The sister has been in touch with a brother [who] appealed for urgent funds stating that his resources are limited. The sister has provide me proofs that have confirmed that the brother is . . . true. . . . I know the sister and by Allah all money will be transferred to her. The sister will then transfer the money to the brother. . . ."

The LaRose indictment also alleges that Mohammed forwarded her a questionnaire "in which [he] asked another woman about her beliefs and intentions with regard to jihad."

At the end of the questionnaire, prosecutors said, Mohammed wrote: "The reason why I am not providing much information as to why I am asking the above-mentioned information is due to security. . . . Also, if you have any contacts to other sisters (only the ones whom you extremely trust. . .!!!), please forward this message to them."

Mohammed and LaRose met in a jihadist chat room, sources said.

The FBI arrested Mohammed on July 6, family members said, and he is being held at the Berks County Youth Detention Center, about three hours' drive from their home, which is near Baltimore.

During several interviews, relatives said family members are all legal residents of the United States, and moved here to get a better education. The parents are the first in the family to be literate; the children will be the first generation to attend college. Mohammed's father works for a delivery company.

Mohammed and his siblings shined in school, but did not socialize much, relatives said, because their parents insisted that they stick to their studies. They were not permitted sleepovers or even to play with other children in the neighborhood.

"School, education is everything," a relative said. "If you waste one second on anything else, you are disrespecting your elders."

A year after they arrived, Mohammed found himself excelling academically, but also, to the concern of other family members, spending hours alone online. He became moody and did not talk much, though he never spoke of violent, religious, or political thoughts - the kind of comments authorities found on his computer.

Mohammed's outlook changed after the FBI seized the family's computers last year and began meeting with him. Mohammed seemed to become more social, relatives said. "We hoped he'd come out of his shell more when he went off to college," a relative said.

Family members remain shocked, they said. They believe Mohammed was lured by an adult and was too young to understand the consequences.

"Some 47-year-old woman was taking advantage of a kid who was just 14 or 15 years old, someone who's easy to brainwash," a relative said. "How did this happen?"

Contact staff writer John Shiffman at 301-320-6655 or jshiffman@phillynews.com.