Unsubstantiated reports of a dirty bomb attack in New York City that sent police into heightened alert this weekend came from a Jerusalem-based Web site that claims more than a million daily readers but is criticized by intelligence experts as unreliable.

Giora Shamis and his wife, Diane Shalem, launched DEBKAfile (Debka.com) in 2000 to focus on security, terrorist activities and military affairs in the Middle East. Shamis claims he and his wife predicted the Al Qaeda attack on the World Trade Center.

Shamis says Debka, which is financed by subscriptions and ad revenue, has a daily readership of 1.3 million. But its reports, published in English and Hebrew, rely on anonymous sourcing and often prove untrue.

"I don't know a single Israeli official who sees the site as a reliable source," said Ronen Bergman, an investigative reporter for Israel's Yediot Ahronot daily who specializes in espionage.

Last week, the site reported intercepting Al Qaeda messages on radioactive attacks against American cities, identifying New York, Los Angeles and Miami as potential targets.

"We have about four or five people that are sitting around the clock, 24-hours-a-day, tracking the different Al Qaeda publications," Shamis said. "For us, this kind of story is routine."

But the reaction was not.

On Friday, the New York Police Department increased its radiological monitoring and established checkpoints to examine vehicles at various points around Manhattan. NYPD Deputy Commissioner Paul J. Browne said the measures were prompted by a Debka report on online Al Qaeda chatter about a truck packed with radioactive material.

By Saturday, the department had eased its alert, but Shamis stood behind his report.

Debka, named for a traditional Arab dance, claims to have been the first to publish in 2001 that U.S. and British forces were operating in Afghanistan and that Saudi Arabia opposed U.S. use of its military bases. But it has also flopped badly, including a 2001 report that China sent troops to Afghanistan to fight alongside the Taliban.

Shamis concedes some Debka reports haven't panned out, but says his site is as reliable as the mainstream media.

Debka's home page on Sunday featured a report on Iran conspiring with the Syrian army to topple Syrian President Bashar Assad and another on Hezbollah guerrillas in Lebanon acquiring new missiles capable of shooting down Israeli warplanes.

"I don't take Debka seriously as a reliable source ... but there are people who take it seriously and that's what makes it dangerous," said Boaz Ganor, an Israeli anti-terror expert.

If American officials took actions based on a Debka report, then it indicated "panic and unprofessionalism," he said.