New York City police have been on high alert for potential threats to the city since the US raid that killed al-Qaida leader Osama bin Laden over a week ago, though Kelly said the men had no apparent link to al-Qaida.

"We are concerned about lone wolves acting against New York city in the wake of the killing of bin Laden," Bloomberg said. "Those perhaps are the toughest to stop."

Officials refused to give details on how the first undercover officer met Ferhani, who later introduced the officer to Mamdouh.

Ferhani, who had been arrested for a robbery in Manhattan last October at a Midtown hotel, was known to the department through intelligence before the arrest. Police said they shared the information with the Joint Terrorism Task Force, which declined to pursue the case federally.

New York passed its own anti-terrorism law within six days of the September 11, 2001, attacks, but the statute has been rarely used.

"New York City is an international symbol of freedom and liberty, and for that reason we will always be a target," Mayor Michael Bloomberg said. "And we will always be on guard to protect the people of this city."

Authorities said Ferhani is an unemployed aspiring actor who may have worked at swank department stores. They say he moved to the US in August 1995 from Algeria with two siblings and his parents, who claimed asylum. He has been living in Queens and had been granted permanent resident status, but is facing deportation because of some brushes with the law, Gandy said. Details weren't immediately available.

Mamdouh, a tall lanky man who is a native of Casablanca, is a taxi service dispatcher. He came to the United States with his parents in August 1999 and is now a US citizen, officials said. His attorney said he attended a local high school and lives in Queens with his brother and sister, and his parents are local business owners.

He is also facing an unrelated burglary case in Queens, Gandy said.

Extra security had already been in place at local religious institutions since the death of bin Laden. David Pollock, who helps advise synagogues on security and emergency preparedness for the Jewish Community Relations Council of New York Inc., said there has long been a heightened awareness among local Jewish organizations that they could be targeted in terror attacks, citing the 2009 arrest of four people who plotted to bomb Bronx synagogues.

The case recalled another NYPD investigation that resulted in the conviction of a Pakistani immigrant on charges he plotted to bomb the subway station in Herald Square to avenge the wartime abuses of Iraqis.

The suspect, Shahawar Matin Siraj, had caught the attention of a police informant and an undercover officer - both assigned to track Islamic extremists following the September 11 attacks - with his anti-American rants.

After plotting with the informant, Siraj and another man who later became a cooperator against him were arrested on the eve of the 2004 Republican National Convention carrying crude diagrams of the subway station situated below a dense shopping district that includes Macy's flagship department store.

Siraj was sentenced in 2007 to 30 years in prison.