An explosion rocked a crowded Manhattan neighborhood and injured 29 people, and a suspicious device discovered blocks away was safely removed early Sunday. Mayor Bill de Blasio ruled out any terror connections, but called the blast an "intentional act."

"Tonight, New York City experienced a very bad incident," de Blasio said at a news conference near the scene in Chelsea. "We have no credible and specific threat at this moment."

De Blasio tried to calm any fears among nervous New Yorkers, saying the explosion had no terrorist connection and wasn't related to a pipe bomb explosion earlier Saturday in New Jersey that forced the cancellation of a charity run.

"Now, I want to be clear whatever the cause, whatever the intention here, New Yorkers will not be intimidated," the mayor said. "We are not going to let anyone change who we are or how we go about our lives. "

It was unclear who was behind the blast and what motivated it.

A law enforcement official told The Associated Press that a second device that officers investigated four blocks from the scene appeared to be a pressure cooker attached to wiring and a cellphone. The source, speaking on condition of anonymity because the person was not authorized to speak about an ongoing investigation, said the device was found inside a plastic bag on West 27th Street. The device was removed with a robot and taken to a department firing range in the Bronx.

The law enforcement official also said that the explosion that rocked a bustling Chelsea neighborhood appeared to have come from a construction toolbox in front of a building. Photos from the scene show a twisted and crumpled black metal box.

The blast happened on West 23rd street, in front of a residence for the blind, near a major thoroughfare with many restaurants and a Trader Joe's supermarket. Witnesses say the explosion at about 8:30 p.m. blew out the windows of businesses and scattered debris in the area. Officials said no evacuations were necessary.

Fire Department Commissioner Daniel Nigro said several people were taken to hospitals with injuries. One of the injured suffered a puncture wound that was considered serious. He said the other injuries were minor, described as scrapes and bruises.

While police investigated the second site, spokesman J. Peter Donald tweeted a warning to some residents to stay away from windows facing 27th Street.

A number of New York City subway routes were affected by the explosion, which rattled some New Yorkers and visitors on the heels of the 15th anniversary of the 9/11 terror attacks.

Chris Gonzalez, visiting from Dallas, was having dinner with friends at a restaurant in the area.

"We felt it, we heard it, the restaurant went real quiet, the 26-year-old Gonzalez said. "It wasn't like jolting or anything, everyone just went quiet."

Rudy Alcide, a bouncer at Vanity Nightclub at 21st Street and 6th Avenue, said he, at first, thought something large had fallen.

"It was an extremely loud noise, everything was shaking, the windows were shaking, it was crazy," he said. "It was extremely loud, almost like thunder, but louder."

The FBI and Homeland Security officials, along with the ATF arson and explosive task force were also at the scene.

The White House said President Barack Obama has been apprised of the explosion in New York City and will be updated as additional information becomes available.

In St. Cloud, Minnesota, police said multiple people were injured at a shopping mall Saturday evening in an attack that possibly involved both a shooting and stabbing. The suspect was believed to be dead.

Hillary Clinton says she has been briefed "about the bombings in New York and New Jersey and the attack in Minnesota."

She says the nation needs to support its first responders and "pray for the victims."

"We have to let this investigation unfold," she said.

Donald Trump moved ahead of New York City officials when he declared a "bomb went off" in the city before officials had released details. He made the announcement minutes after stepping off his plane during a rally at an airport hangar in Colorado Springs, Colorado.

"I must tell you that just before I got off the plane a bomb went off in New York and nobody knows what's going on," Trump said.

He continued, "But boy we are living in a time -- we better get very tough, folks. We better get very, very tough. 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."

The Republican presidential nominee made the comments around 9:10 p.m., shortly after the explosion and as emergency officials responded to the blast. A spokeswoman for Trump did not respond to an email asking whether he was briefed about the incident before taking the stage.

The reports of the blast came hours after a pipe bomb exploded in Seaside Park, New Jersey, shortly before thousands of runners participated in a charity 5K race to benefit Marines and sailors. The run was cancelled and no injuries were reported.

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Associated Press writers Jake Pearson, Ezra Kaplan and Maria Sanminiatelli in New York and Eric Tucker and Kevin Freking in Washington contributed to this report.