Police investigating a terror attack that could have set off a deadly fireball in Times Square - a major tourist destination - are focused on finding a man who was videotaped shedding his shirt near the SUV where the bomb was found.

Police said the petrol-and-propane bomb was crude but could have sprayed shrapnel and metal parts with enough force to kill pedestrians and knock out windows on one of America's busiest streets, full of Broadway theatres and restaurants on a Saturday night.

The bomb "looks like it would have caused a significant fireball" had it fully detonated, police Commissioner Ray Kelly said.

A large amount of fertiliser rigged with wires and fireworks was found with the bomb, but police said it was not the ammonium nitrate grade that can explode.

The surveillance video shows an unidentified white man apparently in his 40s slipping down an alley and taking off a shirt, revealing another underneath. In the same clip, he's seen looking back in the direction of the smoking vehicle and furtively putting the first shirt in a bag, Kelly said.

The homemade bomb was made largely with ordinary items, including three barbecue grill-size propane tanks, two 5-gallon petrol containers, store-bought fireworks and cheap alarm clocks attached to wires.

"Clearly it was the intent of whoever did this to cause mayhem, to create casualties," Kelly said.

Authorities didn't know how deadly the bomb could have been, how it failed or who was responsible.

Police had already identified the registered owner of the 1993 Nissan Pathfinder - which didn't have an easily visible vehicle identification number and had license plates from another car - and were looking to interview him. Police also were searching more video, believed to be in the possession of a Pennsylvania tourist, of a person spotted near the car.

The bomb at Times Square, one of the flashiest and best-known places on Earth, was found at the height of dinner hour before theatregoers headed to Saturday night shows.

Timers were connected to a 16-ounce (450-gram) can filled with fireworks, which were apparently intended to set the gas cans and propane afire, Kelly said. The vehicle would have been "cut in half" by an explosion, and people nearby could have been sprayed by shrapnel and killed, he said.

Police had feared that another component - a metal rifle cabinet packed with a fertiliser-like substance and rigged with wires and more fireworks - could have made the device even more devastating. Test results late Sunday showed that it was indeed fertiliser - but the New York Police Department's bomb experts believe it was not a type volatile enough to explode like the ammonium nitrate grade fertiliser used in previous terror attacks, said police spokesman Paul Browne.

The exact amount of fertiliser was unknown. Police estimated the cabinet - with a manufacturer-listed weight of 78 pounds (35 kilograms) - weighed 200 pounds (90 kilograms) to 250 pounds (113 kilograms) when they pulled it from the vehicle.

New York's busiest streets, choked with taxis and people on one of the first summer-like days of the year, were shut down for 10 hours, unnerving thousands of tourists attending Broadway show, museums and other city sights. Detectives took the stage at the end of some shows to announce to theatergoers that they were looking for witnesses in a bombing attempt.

"No more New York," said Crysta Salinas. The 28-year-old Houston woman was stuck waiting in a deli until 2am local time because part of a Marriott hotel was evacuated because of the bomb.

A Pakistani Taleban group claimed responsibility for the failed attack in a 1-minute video. Kelly and Mayor Michael Bloomberg, however, said police have no evidence to support the claims, and noted that the same group had falsely taken credit for previous attacks on US soil. The commissioner also cast doubt on an email to a news outlet claiming responsibility.

Bloomberg noted that the investigation was in its early stages but said, "So far, there is no evidence that any of this has anything to do with one of the recognized terrorist organisations."

The NYPD and FBI were also examining "hundreds of hours" of security videotape from around Times Square, Kelly said.

Police released a photograph of the dark-colored SUV as it crossed an intersection at 6:28pm Saturday. A vendor pointed the SUV out to an officer about two minutes later.

The license plate found on the vehicle did not belong to the SUV; police said it came from a car found in a repair shop in Connecticut.

Duane Jackson, a 58-year-old handbag vendor, said he noticed the car and wondered who had left it there in a no-standing zone.

Jackson said he looked in the car and saw keys in the ignition with 19 or 20 keys on a ring. He said he alerted a passing mounted police officer.

They were looking in the car "when the smoke started coming out and then we heard the little pop-pop-pop like firecrackers going out and that's when everybody scattered and ran back," he said.

"Now that I saw the propane tanks and the petrol, what if that would have ignited?" Jackson said. "I'm less than 8 feet away from the car."

Times Square lies about four miles north of where terrorists bombed the World Trade Centre in 1993, then destroyed it on September 11, 2001.

The Pakistani Taleban claimed responsibility in a video posted on the internet on Sunday, according to the SITE Intelligence Group. SITE, a US-based terrorist tracking organisation, first uncovered the video on YouTube; it later appeared to have been removed from the website.

In a copy of the video provided by SITE, an unidentified voice speaking in Urdu, the primary language in Pakistan, says the group takes "full responsibility for the recent attack in the USA." The video does not mention any details about Saturday's attack.

The militant group said the attack was revenge for the death of its leader, Baitullah Mehsud, and the recent slaying of al-Qaida in Iraq leaders Abu Omar al-Baghdadi and Abu Ayyub al-Masri, who were killed by US and Iraqi troops last month north of Baghdad. The video also mentioned Aafia Siddiqui, a Pakistani scientist who was convicted in a US court in New York in February of trying to kill American service personnel after her arrest in Afghanistan in 2008.

If the claim is genuine, it would be the first time the Pakistani Taleban has struck outside South Asia. It has no known global infrastructure like al-Qaida.

In at least one past instance, the Pakistani Taleban has claimed responsibility for an attack it played no role in. Mehsud reportedly said his men were behind a mass shooting in March 2009 at the American Civic Association in Binghamton, New York, in April 2009. That claim turned out to be false.

The last terror threat in New York came last fall when air shuttle driver Najibullah Zazi admitted to a foiled homemade bomb plot aimed at the city subway system.

The theater district in London was the target of a propane bomb attack in 2007. No one was injured when police discovered two vehicles loaded with nails packed around canisters of propane and petrol.

-AP

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