New York ? The tip last year from would-be FBI informant Tanveer Choudhry sounded frightening. Two brothers intent on killing Jews planned to hijack a pair of gasoline trucks and turn the Verrazano Narrows Bridge into an inferno, the Pakistani immigrant told agents.

But before New Yorkers knew anything about it, investigators came to a familiar conclusion: The tipster was lying.

Authorities say that since the Sept. 11 attacks, an untold number of hoaxers have preyed on fears of terrorism on U.S. soil by telling phony stories in hopes of winning favors from the government, settling scores or simply causing mischief.

Most of the tales are quickly and quietly discounted. But this week an alleged al-Qaida plot to bomb the New York subway system highlighted the potential for tips to spread fear and tie up investigators before they are proven to be false alarms.

Hoping to discourage phony threats, Congress passed a law last year making it a crime to convey false or misleading information about a terrorist attack. The offense carries up to five years in prison – and 20 years to life if the hoax causes injury or death.

“Hoaxes waste resources that are needed to chase down real tips,” said Dean Boyd, spokesman for U.S. Immigration and Customs Enforcement, the investigative arm of the Homeland Security Department.

Choudhry was deported after admitting he provided the fake tip about the bridge plot in a bid to win asylum – a common motive. Other tipsters make false accusations “to get back at their ex-wives or someone they were in car accident with. : You name it,” Boyd said.

The recent subway threat apparently was more a case of bad information than outright fabrication, some officials said.

Acting on a tip volunteered to U.S. authorities by a paid informant in Baghdad, city officials and the FBI last week issued a public warning and sent thousands of extra police officers to guard the subways. Four days later, the protection was lifted after investigators found no evidence to support his story that a team of terrorists would use briefcases and baby strollers packed with explosives to attack the subway.

Some news reports have said the threat was a hoax. But some federal officials said it is more likely the informant was passing along false information he believed was true.

Local officials have stood by their decision to boost security, saying that the informant had passed a lie detector test and had a good track record, and that his warning was too detailed and too dire to be ignored.

“This was a planned attack that had a specific time and target and method,” Mayor Michael Bloomberg said this week after the informant’s story unraveled. “It was the first really serious allegation of a direct attack on this city since 9-11.”

Other chilling threats have come and gone without becoming full-blown scares.

Last December in upstate New York, Danian Terzi alarmed the state troopers who arrested him for purse snatching by claiming he and others had smuggled C-4 explosives from St. Louis to New York with plans to detonate a bomb in Times Square on New Year’s Eve. Court papers say that after the FBI was called in, Terzi “recanted the story and admitted the story was an elaborate hoax.”

Ahmed Allali – an Algerian immigrant and Indianapolis gas station owner facing deportation for having a fraudulent passport – claimed last year that he had traveled to the United States in 1998 with al-Qaida operatives, and provided extensive details about a sleeper cell that was planning to set off bombs in five major cities. The scare tied up scores of investigators before Allali admitted making up the tale to avoid being kicked out of the country.