NEW YORK — Police acting without the FBI's knowledge may have inadvertently helped blow the surveillance of a terrorism suspect and compromised a bomb plot investigation at a sensitive stage by questioning an imam about him, a criminal complaint suggests.

"They came to ask me about your characters," the Muslim religious leader, Ahmad Wais Afzali, told Najibullah Zazi in a secretly recorded Sept. 11 telephone conversation. "They asked me about you guys."

At least one of those New York Police Department detectives, referred to in the recently unsealed criminal complaint, works for a division that operates independently from an FBI-run terrorism task force.

The complaint also suggests investigators may have tipped off Zazi, a 24-year-old Denver airport shuttle driver, by towing and searching a rental car he was using on a New York City trip that heightened fears of an attack.

The maneuver, authorities say, produced evidence of bomb-making instructions retrieved from a hard drive on Zazi's laptop.

But it also apparently didn't get by the suspect: In the phone conversation with Afzali, Zazi said the car's disappearance convinced him he was being watched.

NYPD and FBI officials have denied that the potential missteps forced their hand in a series of high-profile raids last week, prompted Zazi to abort his New York visit and caused friction between the two agencies, which work together through the Joint Terrorism Task Force.

Zazi, his father and Afzali were arrested over the weekend on charges they lied to the FBI but weren't charged with terrorism, and the scale and scope of the plot remains unclear. They have denied the charges against them.

Zazi's arrest in Colorado touched off an intense flurry of government terror warnings and advisories, with authorities cautioning transit systems, sports stadiums, hotels and entertainment complexes to be vigilant as investigators search for more suspects in a possible al-Qaida plot to set off hydrogen-peroxide bombs.

Law enforcement officials said Zazi may have been plotting with others to detonate backpack bombs on New York trains in a scheme similar to the attacks on the London subway and Madrid's rail system in the last few years. Backpacks and cell phones were seized in raids on apartments Zazi visited in New York.

In a statement, the FBI and the Department of Homeland Security said that while the agencies "have no information regarding the timing, location or target of any planned attack, we believe it is prudent to raise the security awareness of our local law enforcement partners regarding the targets and tactics of previous terrorist activity."