Slumping in his prison clothes and pallid from a year behind bars, Shahawar Matin Siraj didn’t look like much of a threat as he silently endured a routine hearing in federal court this month.

But the 23-year-old Pakistani immigrant stands accused of a scheme to attack a busy New York subway station with bombs hidden in backpacks.

As police seek to secure the nation’s largest transit system in the wake of the London Underground bombings, they say they are concerned about angry, isolated men like Mr. Siraj as much as organized terror networks like Al Qaeda.

“One of the department’s ongoing concerns is the emergence of ‘lone wolves,'” the New York Police Department’s chief spokesman, Paul Browne, said.

The first known plot against New York’s subways was averted in 1997, police said, when officers acting on a tip burst into Palestinian-born Gazi Ibrahim Abu Mezer’s Brooklyn apartment and shot him in the leg as he reached for a toggle switch on a pipe bomb. He was sentenced to life in prison after testifying he wanted to kill Jews riding the subway in Brooklyn.

Despite initial reports of Hamas ties, Mezer was acting with a single alleged accomplice, who was convicted of an immigration violation and deported after three years in prison.

Mr. Siraj was working at an Islamic bookstore in Brooklyn when he was approached in 2003 by an Egyptian-born police informant. The informant spent months secretly monitoring Mr. Siraj and his co-defendant, James Elshafay.

As a result, police say they have recordings of the two men and the informant discussing how attacks on three spots – the Verrazano Narrows Bridge and subway stations at Herald Square and next to Bloomingdale’s on Manhattan’s East Side – could damage the economy as part of a holy war against America.

Messrs. Siraj and Elshafay were arrested last August and charged with conspiring to damage the Herald Square station, a charge with a maximum 20-year sentence. Defense attorney Khurrum Wahid described Mr. Siraj as a hardworking immigrant entrapped by an informant who whipped his client into a rage over abuses against Muslims like the scandal at Abu Ghraib prison in Iraq, Mr. Wahid said.

“He manages to convince them that they need to do something,” Mr. Wahid said. “He puts the idea of attacking the United States into their head.”

Mr. Elshafay has stopped appearing at court hearings, and Mr. Wahid said he believes the 20-year-old man is cooperating with authorities. Besides the 1993 World Trade Center bombing and the terrorist attack of September 11, 2001, the nation’s most significant terrorist plots and attacks were by men acting alone or in pairs without ties to known radical networks, a terrorism expert at the Rand Corporation, Bruce Hoffman, said.