NEW YORK — Authorities are investigating three incidents – explosions in New York and New Jersey and a stabbing attack in Minnesota – that happened within 12 hours Saturday and sowed fears of terrorism.

Officials said they could identify no definitive links between the disturbances – a bombing that hurt 29 in Chelsea, an explosion along the route of a scheduled race in Seaside Park, New Jersey, and a stabbing that wounded nine in a St. Cloud, Minnesota, mall.

A mangled toolbox sits at the scene of an explosion Saturday night in the Chelsea neighborhood of New York.

FBI agents carry on their investigation Sunday at the scene of Saturday's explosion at West 23rd Street and Sixth Avenue in Manhattan's Chelsea neighborhood in New York.

But each incident raised the possibility of terrorist connections, prompting federal and local law enforcement to pour resources into determining exactly what happened and why.

A news agency linked to the Islamic State claimed Sunday that the suspect in Minnesota, who was fatally shot by an off-duty police officer, was “a soldier” of the militant group, though there was no confirmation of what connection the man may have had.

A claim of responsibility is no guarantee that the terrorist group directed or even inspired the attack, and authorities said they were still exploring a precise motive. The terrorist group made no similar claims about the New York and New Jersey incidents.

In New York, authorities said there was no evidence that the Saturday-night explosion was motivated by international terrorism, though they confirmed that the bombing was intentional. “This is the nightmare scenario,” said Gov. Andrew Cuomo.

Cuomo said nearly 1,000 police officers and National Guard troops would be sent to bus stops, train stations and airports, as investigators with the New York Police Department, the FBI, and the Bureau of Alcohol, Tobacco, Firearms and Explosives worked to identify the person or people responsible for the explosion.

Late Sunday, law enforcement officials said they were still looking to see if the explosions in New York and New Jersey were related. Investigators were aggressively searching for connections between the two incidents, said one federal law enforcement official, who cautioned that as of Sunday afternoon authorities had not tied them together definitively.

A second law enforcement official said that while it looked like the New Jersey and New York blasts “might be connected,” investigators still didn’t have any hard evidence. The official said that only one of the three pipe bombs in New Jersey detonated.

Those injured in the Saturday-night blast in Chelsea had been released from hospitals by Sunday.

The Manhattan explosion occurred about 8:30 p.m. in the area of West 23rd Street between Sixth and Seventh avenues, injuring 29 people as it hurled glass and debris into the air, officials said. Surveillance video showed passersby running to get away from the blast, and investigators said they would comb through that and older footage to try to identify those responsible.

Authorities said the explosion was produced by some type of bomb, and they posted on Twitter a photo of what appeared to be a mangled Dumpster or garbage container. Masum Chaudry, who manages a Domino’s Pizza near the scene, said the explosion “shook the whole building” and caused “total chaos.”

A short time after the explosion, just a few blocks away, police found another potentially explosive device, which looked like a pressure cooker with wiring. Both that device and the remnants of that which exploded will be sent to the FBI’s lab in Quantico, Virginia, for analysis, authorities said. Pressure cookers were used in the two bombs detonated at the Boston Marathon in 2013.

Sara Miller, who was at a restaurant two blocks from the explosion, said she heard the blast, then saw people scrambling to get away. “I was here on September 11th so I thought, maybe, you know, I was being paranoid . . . but then I saw people running,” said Miller, 42. “It is a scary time because you never know when it will happen again.”

Officials differed on whether to call the Saturday night explosion an act of terrorism. Cuomo said: “It depends on your definition of terrorism. A bomb exploding in New York is obviously an act of terrorism, but it’s not linked to international terrorism.”

City, police and FBI officials said it was too early to determine any type of motivation, although they insisted they would not shy from labeling the crime an act of terror if it became appropriate to do so.

“We do not know the motivation. We do not know the nature of it. That’s what we have to do more work on,” said New York City Mayor Bill de Blasio.

The incident comes as foreign leaders, including many heads of state, are heading to Manhattan for the United Nations General Assembly. Secretary of State John Kerry arrived Saturday, while President Obama is scheduled to head to the city Monday.

This annual meeting – held more than two miles from the site of the explosion in Chelsea – is traditionally a challenging time for New York, as many roads are shut down and the heavy security leads to traffic jams.

Officials said they had already prepared to beef up security, and now they would intensify those efforts.

The Democratic and Republican presidential candidates offered varied reactions to news of the incident.

As early reports circulated Saturday night, Donald Trump declared that a “bomb went off” in New York City and said: “We better get very, very tough. We’ll find out. It’s a terrible thing that’s going on in our world, in our country and we are going to get tough and smart and vigilant. … We’ll see what it is. We’ll see what it is.”

Hillary Clinton condemned what she characterized as the “apparent terrorist attacks” in Minnesota, New Jersey and New York.

“This should steel our resolve to protect our country and defeat ISIS and other terrorist groups,” Clinton said, using an acronym for the Islamic State. She added, “I have laid out a comprehensive plan to do that.”

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