NEW YORK (Reuters) - Investigators are treating a car bomb defused in New York’s Times Square as an attempted terrorist attack but have no evidence to support a Taliban claim of responsibility, police said on Sunday.

Police are poring over surveillance camera footage and a device made of propane, gasoline and fireworks after officers, alerted by a street vendor, found the bomb in a vehicle on Saturday evening as Times Square was packed with tourists and theater-goers.

U.S. Homeland Security Secretary Janet Napolitano called the scare a “potential terrorist attack” but she and other officials held off saying whether there was a link to Islamist groups or to a domestic cause in the United States.

A U.S. intelligence official told Reuters “we just don’t know at this point who may be behind this event.”

“Either way we should know more soon,” said Paul Rogers, a terrorism expert at Britain’s Bradford University. “Because the explosives didn’t go off, the forensics experts have a large amount of material in the vehicle to work on.”

The Taliban in Pakistan said it planted the bomb to avenge the killing in April of al Qaeda’s two top leaders in Iraq as well as U.S. interference in Muslim countries.

But New York Police Commissioner Raymond Kelly said there was “no evidence” to support the Taliban’s claim.

“If this had detonated, in my judgment it would have caused casualties, a significant fireball,” he said. “A terrorist act doesn’t necessarily have to be conducted by an organization. An individual can do it on their own.”

Michael Cheah, senior portfolio manager at SunAmerica Asset Management, said the car bomb was an “isolated incident” that was not likely to spark any Treasury market reaction.

New York and its 8 million people have been on high alert since the September 11 attacks in 2001 when airliners hijacked by al Qaeda militants toppled the World Trade Center’s twin towers, killing more than 2,600 people.

Last year, police said they thwarted a plot to bomb the New York subway system. Two men have pleaded guilty in that case.

‘WE CAN’T RULE ANYTHING OUT’

“It’s worth recalling the trend in radicalization in the USA,” said Henry Wilkinson, senior intelligence analyst at London-based security company Janusian. “It does not look like a prototypical al Qaeda attack. But again it’s early days.”

The deadliest home-grown attack in the United States killed 168 people in 1995 when a fuel-and-fertilizer bomb planted in a truck by Timothy McVeigh and another right-wing extremist exploded at a federal building in Oklahoma City.

Kelly said a white man in his 40s had been identified in security footage and was seen removing a dark shirt to reveal a red shirt about half a block from where the vehicle was left with its engine running and hazard lights flashing.

Kelly said it was not clear whether the footage was related to the bomb and that another claim of responsibility emailed to a local news organization was also being investigated.

“We can’t rule anything out at this time,” he said when asked whether the Viacom Inc office in Times Square was a possible target after the satirical cartoon “South Park” depicted a controversial image of the Prophet Mohammed.

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“South Park” is shown on the Comedy Central network owned by Viacom.

Times Square was evacuated on Saturday evening but the entertainment and shopping area in midtown Manhattan largely reopened on Sunday with a heavy police presence. Broadway officials said all shows did go on, some beginning late.

The bomb squad removed three propane tanks, consumer grade fireworks, two full five-gallon (19-liter) gasoline cans, two clocks, electrical wire and other components from the vehicle, a dark green Nissan Pathfinder.

On Sunday, the bomb squad blew open a gun locker that was also in the vehicle and found eight bags of an “unknown substance” with “the look and feel” of fertilizer, Kelly said.

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The Connecticut license plate found on the rear of the Pathfinder came from another vehicle that was now in an auto repair shop in Connecticut, Kelly said.

THE REAL DEAL

“We’re going to do whatever is necessary to protect the American people, to determine who’s behind this potentially deadly act and to see that justice is done,” President Barack Obama told a news conference.

Similarities between the attempted attack in Times Square and another in London in 2007 included a vehicle abandoned in a crowded area with the intent of causing mass casualties, said Sajjan Gohel, director for international security at the London-based research company Asia Pacific Foundation.

“Was this done by an established group or, as in the London case, by self-radicalized people?” Gohel said.

He also voiced skepticism about the Taliban claim and said video footage could be the key for investigators.

Napolitano told ABC News there was no evidence the incident in Times Square was “anything other than a one-off” and that the bomb “doesn’t look like it is a very sophisticated one.”

New York police said they searched transit hubs, landmarks and other sensitive areas after the bomb was discovered but did not find anything suspicious.

“We have no idea who did this or why,” said New York Mayor Michael Bloomberg. He said a T-shirt vendor noticed “an unoccupied suspicious vehicle” and alerted a policeman on horseback, who saw the Pathfinder had smoke coming from vents near the back seat and smelled of gun powder.

“This wasn’t make believe. This wasn’t a false alarm,” said New York Fire Commissioner Sal Cassano. “This was the real deal -- to hurt people.”