A prayer service inside the mosque in Selden, N.Y., where the man linked to Al Qaeda, Bryant Neal Vinas, is said to have prayed.
He grew up in the solid middle class of Suffolk County, the son of an engineer, the child of a couple that had emigrated from South America, a fan of football and video games, an altar boy and, eventually, a Boy Scout.
But Bryant Neal Vinas became very angry, according to his mother, when his parents’ bitter divorce cleaved through his adolescence. He opted against college and instead joined the United States Army at 18.
Years later, he became a Muslim, joined a mosque, began visiting jihadist Web sites and, in 2008, found himself traveling to Pakistan, and eventually Afghanistan. There, this young man from an American suburb tried to kill American soldiers in a Qaeda rocket attack against a military base, according to federal court papers unsealed in Brooklyn on Wednesday.
Some two months after the September assault, Mr. Vinas, going by names like Bashir al-Ameriki, or Bashir the American, was picked up by Pakistani authorities in Peshawar, according to American officials. Days later, he was back in the United States, providing counterterrorism officials with what they called “valuable information” gleaned from his visits to Al Qaeda’s camps, leading to the arrests of senior Qaeda operatives and to Predator drone strikes.
Mr. Vinas, 26, pleaded guilty in January to conspiring to murder United States nationals, providing material support to Al Qaeda and receiving military training from the group.
He also told Brooklyn federal prosecutors and F.B.I. agents about discussions he had with operational planners from Al Qaeda about a plot to blow up a Long Island Rail Road train inside Pennsylvania Station, according to several law enforcement officials.
The information prompted a flurry of security activity over the Thanksgiving holiday as the authorities scrambled to take extra precautions, though it did not appear the planned attack had yet been put into motion.
The slight, dark-haired and pale-skinned Mr. Vinas, who the officials said began formally cooperating with federal authorities about two months later, also admitted assisting Al Qaeda by providing “expert advice and assistance” that was “derived from specialized knowledge of the New York transit system and the Long Island Rail Road,” according to the court papers.
Two officials said that Mr. Vinas, who lived in Patchogue until he went to Pakistan, learned about the Long Island Rail Road as a regular rider and shared that information with Qaeda leaders in Pakistan, who had planned to use it in an attack. But neither official would provide specifics, and it appeared that Mr. Vinas’s knowledge of details of the planned attack may have been limited. The officials, like others interviewed for this article, spoke on the condition of anonymity because the investigation is continuing.
The papers unsealed on Wednesday include the terse, three-count criminal information that lists the charges against Mr. Vinas. Filed by prosecutors in the office of the United States attorney in Brooklyn, Benton J. Campbell, it says that Mr. Vinas received “military-type training” from and on behalf of Al Qaeda between March and August 2008.
He pleaded guilty to all three counts on Jan. 28 before Judge Nicholas G. Garaufis of United States District Court in Brooklyn.
A lawyer for Mr. Vinas, who the court papers say was also known as Ibrahim and Ben Yameen al-Kanadee, would not discuss the charges against him or his cooperation with the authorities in the United States or Europe. “We would just ask the public to withhold judgment until all the facts come out in this case,” said the lawyer, Len H. Kamdang.
One United States law enforcement official also said that Mr. Vinas, one of a tiny group of American citizens who have traveled to Pakistan to train with Al Qaeda, had had contact with high-level members of the group there, but provided no names.
For the Qaeda leaders in Waziristan along the Afghanistan-Pakistan border where Mr. Vinas trained, finding an American-born United States citizen with no criminal record and no previous ties to Islamist groups would seem priceless. He would be able to travel freely through the United States and Europe, with a knowledge of New York’s transit systems.
At the same time, Mr. Vinas’s cooperation is nearly as valuable a find for Western intelligence and counterterrorism investigators. He had knowledge of several camps he visited, the network that led him there and the high-level Qaeda officials with whom he met.
Mr. Vinas worshiped at a mosque on Long Island, where he worked briefly as a truck driver and in a car wash, according to officials, one of whom said he had been largely “self-radicalized.” This official said that Mr. Vinas had met some people at the mosque, the Islamic Association of Long Island, but largely turned toward jihad on the Internet.
The mosque’s imam, Nayyar Imam, said Mr. Vinas, whom he called “a very private person,” had been praying there for a little more than a year, but stopped coming a year to 18 months ago.
“I never saw any anger,” he said, calling him “very quiet and very smiley.”
The young man’s father, Juan Vinas, told The Los Angeles Times that his son left home suddenly in September 2007, saying he wanted to study Islam and Arabic. About a year later, F.B.I. agents interviewed the family after a truck bomb killed 55 people in Islamabad. Mr. Vinas has also provided information to European counterterrorism investigators, according to officials, who spoke on the condition of anonymity because of the nature of the case.
The cases in Belgium and France center on two groups of French and Belgian citizens, several of whom trained in the camps, as well as a Moroccan-born woman, Malika El Aroud, who has been accused of using the Internet to recruit the young Muslim men to train with Al Qaeda.
Mr. Vinas is expected to be a key witness in those cases because he spent time in the training camps with the men.
Mr. Vinas, who is in custody in New York, has also provided a 20-page witness statement that will be entered into evidence in the Belgian case, one law enforcement official said.
Ms. El Aroud, a Belgian citizen, has become one of the most prominent Internet jihadists in Europe. Her husband killed the anti-Taliban resistance leader Ahmed Shah Massoud two days before the Sept. 11, 2001, terrorist attacks at the behest of Osama bin Laden.
Since the Sept. 11 attacks, a small number of United States citizens are known to have traveled to train or fight with Al Qaeda. John Walker Lindh of San Francisco was taken into custody by the United States military in Afghanistan, where he had been with the Taliban.