This story was reported by Lisa Irizarry, Alison Fox, Nicole Fuller, Anthony M. DeStefano, Robert E. Kessler, Tom Brune, Maria Alvarez, Ivan Pereira and John Asbury.

Investigators are reviewing a surveillance video showing a person around the device that detonated Saturday and sent shrapnel flying into a Chelsea neighborhood.

Sources familiar with the investigation said Sunday that police will be using facial recognition software and other technology to help assess the image on the video.

“What this guy’s role is is not crystal clear,” one of the sources said.

The bombing led Gov. Andrew M. Cuomo to send 1,000 National Guard and State Police officers to increase patrols in the city. NYPD officers, already on alert for this week’s meeting of the United Nations General Assembly, also are increasing patrols, officials said.

Cuomo said at a Sunday news conference it’s too early to say whether the explosion, as well as the discovery of what appears to be a pressure cooker with a cellphone attached a few blocks away, could be linked to international terrorism, but “a bomb exploding in New York is obviously an act of terrorism.”

The bomb detonated about 8:30 p.m. Saturday in or near a trash bin at 131 W. 23rd St., officials said. No person or group has claimed responsibility.

Mayor Bill de Blasio said at a Sunday news conference that police have been examining the second device, which was discovered and removed before it could explode, to determine if the two are linked. The second device found on 27th Street was similar to homemade bombs that exploded at the Boston Marathon in 2013, killing three people and injuring more than 260.

Sources close to the investigation said the Chelsea bomb also is being studied for any link to another that exploded Saturday morning in Seaside Park, New Jersey, based on the similarity of the detonators used and that the explosives were not complex.

“It’s going to take a lot of serious investigation to get to the facts and the truth,” de Blasio said.

Nassau County Executive Edward Mangano and acting Police Commissioner Thomas Krumpter said in a statement that although there are no known imminent threats to Nassau, “police are monitoring these attacks along with federal, state, and local authorities.”

Suffolk Police Commissioner Timothy Sini said his department is in contact with the NYPD and FBI and that officials are monitoring the situation.

All of the 29 people injured in the explosion have been released from hospitals, officials said.

“As of this time, the exact cause of this explosion has not been determined,” NYPD Commissioner James O’Neill said in a statement Sunday night. “Investigation into this incident is active at this time and an extensive search is being conducted. The area around the explosion site is being treated as a crime scene.”

De Blasio, who toured the scene of the explosion with Cuomo Sunday, said he was reluctant to call the bombing — which is being investigated by the NYPD’s anti-terror unit, the FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives — an act of terror.

“Here’s what we know: It was intentional. It was a violent act. It certainly was a criminal act. It was a bombing,” the mayor said. “To understand any specific motivations, clinical motivations, any connection to an organization, that’s what we don’t know. I think it’s important to say what we know.”

Sources said authorities were going to the different hospitals where victims were taken so they could recover bits and pieces of the shrapnel fragments embedded in their bodies for analysis. The remnants of the explosion were taken to the FBI lab in Quantico, Virginia for analysis and the pressure cooker was taken by the bomb squad to the NYPD gun range at Rodman’s Neck, where it was taken apart, sources said. It also will go to the FBI lab for analysis.

Rep. Peter King (R-Seaford), chairman of the House counterterrorism subcommittee, said it was too early to rule out a connection to international terrorism, even though the bomb breaks the mold by being placed in a location that was not a typical target, such as a major corporation or a government building.

King, who also toured the blast site Sunday, said he doesn’t think the explosive was some “small homemade, little device” because the blast broke several windows and “the dumpster was huge and it was blown halfway down the street.”

Ramon Lopez said early Sunday that he was standing on West 23rd Street near Sixth Avenue Saturday night when he heard the explosion.

Lopez, 48, of East Harlem, said he saw people bleeding, with small pieces of metal fragments stuck in their arms. He described many of the injured as “disoriented” and said he helped guide one bleeding woman to an ambulance as he calmed others.

“What was going through my mind? That another bomb could explode and we’re all going to be gone. But at that time, when the detectives told me I had to leave, I said, ‘You know what, I’m not scared to die, so I need to be here. I need to help, too.’ ”

The explosion blew the construction trash container across the busy street of restaurants, bars, hotels and apartments, knocked glass out of windows and sent people running into the street.

Nita Patel, 36, who lives next to where the explosion occurred, said she was in TriBeCa at 8:30 Saturday night when her neighbor called.

“My neighbor called and said, ‘Did you feel the shake?’ We drove up and parked the car in the garage and then cops said ‘you’re not going back up.’ ”

Patel and her family, including two young children, stayed at a friend’s home on West 22nd Street. They waited Sunday morning to get back into their home.

“It kind of reminds me of Hurricane Sandy,” Patel said, referring to being displaced and scrambling to find somewhere to go.

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