NEW YORK (Reuters) - Investigators on Sunday sifted through blast remnants, examined video and scoured the scene of an explosion that wounded 29 people in Manhattan, attempting to establish if there were any links to international terrorism.

The explosion on a commercial and residential street in New York City’s Chelsea district on Saturday night sent a deafening roar and a powerful shock wave through several blocks, wounding people with shrapnel and flying glass. All 29 victims were released from the hospital, officials said on Sunday.

Mayor Bill de Blasio pleaded for any witnesses to provide tips and promised a security presence that would be “bigger than ever” for the United Nations General Assembly bringing together world leaders in Manhattan for six days starting on Tuesday.

With so little known about the attack, officials said they would deploy an additional 1,000 state police and National Guard to sensitive areas such as transportation hubs.

Federal Bureau of Investigation investigators will examine remnants of the bomb plus an unexploded device found four blocks away as well as a pipe bomb that exploded about 80 miles (130 km) away in New Jersey on Saturday to see if they were connected, officials said.

Police recovered video from both scenes in Manhattan including images of the explosion itself, New York Police Commissioner James O’Neill said.

New York police, the FBI and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives converged on the site for their first daylight view of the site of the explosion, cordoning it off and placing dozens of evidence markers on the ground. Police closed several surrounding blocks to traffic.

“We are in the middle of a very complex post-blast investigation,” O’Neill said.

Although no international group had claimed responsibility, New York state Governor Andrew Cuomo said detonating a bomb in New York City “is obviously an act of terrorism.”

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But the mayor resisted on Sunday when reporters pressed him to call the blast an act of terrorism, saying investigators had yet to determine if there was a political motivation. There were no obvious political targets on the block.

“It was intentional. It was a violent act. It was a certainly a criminal act. It was a bombing. That’s what we know,” said de Blasio, flanked by high-ranking officials of the FBI and the city police and fire departments.

“It could have been something personally motivated. We don’t know yet,” de Blasio said.

MINNESOTA ATTACK

The blast happened the same day as a knife attack at a mall in central Minnesota when a man wounded nine people before he was shot to death by an off-duty police officer. The assailant was a “soldier of the Islamic State,” the militant group said on Sunday.

In a statement on Sunday, Democratic presidential nominee Hillary Clinton said: “I strongly condemn the apparent terrorist attacks in Minnesota, New Jersey, and New York.”

Her Republican opponent, Donald Trump, referring to the Manhattan blast, said on Saturday: “We better get very tough, folks.”

The mayor and the governor both promised that New Yorkers would not be cowed and that apart from the street closures, life would continue as normal. New Yorkers who endured the Sept. 11 attacks in 2001 and devastating Superstorm Sandy in 2012 said they were generally unperturbed.

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A sweep of the neighborhood following the blast turned up another device four blocks away consisting of a pressure cooker with wires attached to it and connected to a cell phone.

Pressure cooker bombs were used in the 2013 Boston Marathon bombing that killed three people and wounded more than 260.

The New Jersey explosion came from a pipe bomb, officials said. A U.S. official familiar with information circulating inside the government said the motive remained unknown and insufficient evidence had been gathered to link the two New York bombs. There was no evidence to connect them to the New Jersey blast, said the source, who spoke on condition of anonymity.

“Almost anybody could have fabricated these bombs and used cellphones as timed detonators,” said another U.S. official familiar with the inquiry. “There are instructions all over the internet, and the crudity, positioning, and relative ineffectiveness of these does not suggest that a more sophisticated group played any role in this.”

The FBI would examine all three devices at its special crime laboratory in Quantico, Virginia, Cuomo said, but city officials said the police bomb squad still had possession of the unexploded device as of midday on Sunday.