Armed with surveillance images and a sport utility vehicle filled with evidence, authorities Sunday began a massive manhunt for whoever tried to set off an explosion large enough to cause mass casualties in Times Square on Saturday evening.

Authorities said Sunday there was no evidence that international terrorist networks were involved in the attempted bombing, although Police Commissioner Ray Kelly wouldn't rule out their involvement.

Instead, New York City police were interested in locating a white man in his 40s  captured by a surveillance camera changing his shirt and furtively looking over his shoulder toward a Nissan Pathfinder that began emitting smoke a few minutes later on West 45th Street

The man was in Shubert Alley, a private walkway connecting theaters about a half-block from the SUV. Late Sunday night, the NYPD released the video to the news media and asked the public to call 800-577-TIPS if they had any information about the man.

"He stops, takes a dark shirt off, looks around, takes that shirt, puts it in a bag and continues to walk south, looking a couple of times in a furtive manner," Kelly said at a news conference Sunday. "It could be perfectly innocent, but we're asking for that individual to come forward."

The car bomb attempt was aimed at the heart of New York's tourist center on a hot spring evening during the theater district's preshow dinner hour. Laden with three propane tanks, dozens of firecrackers, two containers of gasoline and nonexplosive fertilizer, the SUV was parked sometime around 6:28 p.m. with its hazard lights flashing on West 45th Street west of Broadway, an area swarming with pedestrians.

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"It looks like it would have caused a significant fireball," Kelly said, adding: "The vehicle itself would have been cut in half. And you have large number of pedestrians in that area. We were lucky that it didn't detonate."

A Times Square vendor alerted a mounted police officer, Wayne Rhatigan of Holbrook, to popping noises and white smoke pouring from the Pathfinder. At 6:34 p.m., Rhatigan told authorities of the suspicious vehicle, and the area was soon flooded with more police, firefighters and emergency service units.

Streets, part of hotel cleared

Thousands of tourists were evacuated from the streets in a 12-block area around Times Square, police said. Some guests at the Marriott Marquis with rooms facing 45th Street were told to leave the building.

Investigators had recovered the SUV vehicle identification number and determined it had a license plate registered to another vehicle at a repair shop in Connecticut. Kelly said police had identified the owner of that vehicle but declined to name him.

Authorities said there was no intelligence presaging the attempted attack. On ABC's "This Week," Homeland Security Secretary Janet Napolitano said the information authorities have now pointed to a "one-off," or an attacker working independently. Homeland Security officials and the FBI were also investigating.

President Barack Obama praised New York police for quickly acting on the threat and vowed that federal authorities would continue to work closely with local officials.

"We're going to do what's necessary to protect the American people, to determine who's responsible," Obama said.

Times Square has been a terrorist target for decades, officials said. In December, a parked van without license plates prompted police to shut down the area, though no connection to terrorism was found.

"We know we live in a dangerous world," Mayor Michael Bloomberg said at a news conference Sunday evening. "New York has been a target since 1993 when we had the bombing of the World Trade Center."

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Saturday's failed bombing didn't deter Times Square's dense crowds, which were again swarming the scene Sunday.

Regina Kenter, 45, an administrative assistant from Riverhead, her husband and 10-year-old son were among the hordes of pedestrians, keeping plans to see a matinee of the musical "The Addams Family."

"We are typical New Yorkers," Kenter said. "We don't let anything get in the way. That's what they want. They want to disrupt America's way of life."

Extra police to patrol area
Kelly said additional patrol officers would be out Monday to "increase the comfort level" on the streets.

Times Square's intense crowds and prevalence of security cameras present copious amounts of evidence to slog through as investigators work to identify the bomb suspect.

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Police were poring over hundreds of hours of surveillance footage from 82 security cameras stationed around Times Square, Kelly said, and more footage would be obtained Monday from businesses closed over the weekend.

So far, nothing showing the suspect exiting the Pathfinder had been found, Kelly said. One possible lead came from Pennsylvania, where detectives traveled Sunday to meet a tourist who said he may have the suspect on video.

"He believes he may have gotten a picture of someone lunging or leaving the area," Kelly said.

The Pathfinder was taken at 7:30 a.m. to an NYPD forensic lab in Queens, where investigators found a system behind the front seat that Kelly called "workable" with materials that could be found "in any supply store."

On the backseat were two yellow alarm clocks with wires coming out, police said. One clock was connected by wiring to a 16-ounce can full of 20 to 30 M-88 firecrackers, illegal in New York. The can was nestled between two 5-gallon containers of gasoline.

The other clock had wires leading to a long cardboard box in the trunk containing a home kitchen pressure cooker filled with M-88s and a metal rifle case, containing eight bags of nonexplosive fertilizer.

Although investigators were still studying the explosive material, Kelly said the firecrackers apparently were intended as detonators for the rest of the flammable material in the vehicle.

Key questions about the Times Square bombing attempt

Who is responsible? Officials said Sunday that they don't know. Possible suspects range from a "lone wolf" who isn't tied to any terror group to the Taliban or al-Qaida.

Have investigators ruled out any person or terrorist group? No.

Has any group claimed responsibility? Yes, news reports said the Taliban claimed responsibility. The NYPD said an individual e-mailed a news organization and also claimed responsibility. The caller wasn't related to any known terrorist organization, police said.

Do police have any relevant video? Police said they have video of a man seen entering Shubert Alley, taking off a shirt which he wore over a different one and walking away. They are interested in talking with him. A tourist from Pennsylvania also said he has video of a man leaving the area near the car. Surveillance video also captured the car as it entered the area. Cops will be reviewing more video images caught on security cameras.

Has anyone been arrested? Not as of late Sunday.

Do police know who owned the SUV that was packed with the explosive device? Investigators know the Pathfinder's vehicle identification number and determined it had a license plate registered to another vehicle at a repair shop in Connecticut.