An explosion that rocked a crowded Manhattan neighborhood Saturday night and injured 29 people has been determined to be an "intentional act" — but has not been linked to terrorism — New York City mayor Bill de Blasio said.

Less than three hours after the blast, an object police described as a "possible secondary device" was found just a few blocks away from the original explosion. Three law enforcement sources described the find as a pressure cooker with items attached.

It was later sent to an NYPD range in the Bronx aboard a special containment vessel, the mayor's office said

The explosion on West 23rd Street between Sixth and Seventh Avenues in Chelsea was reported at around 8:30 p.m.

Eleven people were taken to Bellevue Hospital for treatment but were released by 8:30 a.m. ET on Sunday morning, according to a hospital spokeswoman.

"The initial indication is this was an intentional act," de Blasio told reporters at a late-night press conference. "There is no evidence at this point of a terror connection to this incident."

There was also "no specific and credible threat against New York City at this point in time from any terror organization," the mayor added.

Police had said the blast in the Chelsea neighborhood of Manhattan appeared to come from inside a large trash bin. Photos on social media showed what looked like a bin that had been mangled in the explosion, although NYPD Commissioner James O'Neill said the blast was believed to have occurred on the street.

"We do have video, and we see the explosion," he said, adding that no arrests had been made.

The FBI Joint Terrorism Task Force was on the scene. The federal Bureau of Alcohol, Tobacco and Firearms said a New York arson and explosives task force had responded to the blast.

The "possible secondary device" was found at 27th Street between Sixth and Seventh Avenues, the New York Police Department said on Twitter shortly after 11 p.m. ET.

Three law enforcement sources told NBC News investigators at the second location were examining what appeared to be a pressure cooker with "tape, wires and a cell phone" left on the sidewalk. The bomb squad was investigating although it had not been confirmed that the object was an explosive device.

'Entire apartment shook'

"There was a loud boom and our entire apartment shook," said Neha Jain, 24, who lives on West 23rd and Sixth Avenue. "All the pictures fell to the floor and then I heard people screaming."

Jain said the explosion shattered the glass in her building's lobby. "My first thought was it's a bomb," Jain said. "It's quite terrifying."

A large police presence, including the New York Office of Emergency Management and the NYPD's counterterrorism unit responded to the scene.

De Blasio also said the investigation has found no link to an explosive device that blew up in a Jersey Shore community earlier Saturday, and which occurred near a planned race route.

In that incident in Seaside Park, a device consisting of three pipe bombs exploded along the planned route of a 5K Marines charity race. No one was hurt. The explosive device was placed in a trash can in that incident, NBC New York reported.

Democratic presidential candidate Hillary Clinton said she was briefed about the explosion as well as the explosive device in New Jersey, but said she would have more to say on the New York incident until the more is known. "Obviously, we need to do everything we can to support our first responders. Also to pray for the victims," she said.

Her Republican rival, Donald Trump, mentioned the explosion at the start of an event in Colorado Springs, and said a "bomb" went off in New York. "Just before I got off the plane a bomb went off in New York, and nobody knows exactly what's going on, but boy, we are living in a time," Trump said. "We better get very tough, folks."

Clinton also called the incident in New York a "bombing" when speaking to reporters Saturday night. New York officials have not said the explosion was a bomb.