NEW YORK - New York City Mayor Bill de Blasio says people will have to patient to get the full story behind Saturday night's bombing in Manhattan that injured 29 people in one of the city's busiest neighborhoods.
"We have more work to do to be able to say what kind of motivation was behind this. Was it a political motivation? A personal motivation...we don't know yet."
A law enforcement official told the Associated Presses that investigators are not putting much stock into a claim of responsibility posted earlier Sunday on the Tumblr social media website.
Tumbler has since taken down the post and has not made any comments.
New York Governor Andrew Cuomo said the blast was an "act of terrorism," but said there is no evidence of "international terrorism."
Cuomo said another 1,000 police and National Guard troops will patrol New York's subway system "just to err on the side of caution."
Meanwhile, all 29 people hurt in the blast are out of the hospital.
Republican presidential candidate Donald Trump tweeted his "warmest regards, best wishes and condolences to all of the families and victims of the horrible bombing." He had earlier said it is time to get "tough, smart, and vigilant."
Democrat Hillary Clinton strongly condemned the New York bombing along with other "apparent terrorist attacks" Saturday in Minnesota and New Jersey.
"I am confident we will once again choose resolve over fear," she said.
U.S. Homeland Security Secretary Jeh Johnson again reminded people that they can also fight terrorism by reporting anything suspicious to police, repeating the slogan "if you see something, say something."
Saturday's apparent bomb exploded outside a building in New York's Chelsea neighborhood, where some of the city's most popular restaurants and bars are located.
The blast left the streets covered with glass and debris. A closed circuit television picture showed windows being blown out of storefronts and parked cars while pedestrians ran for safety.
It is unclear why the Chelsea neighborhood was targeted.
Much of Manhattan has been especially busy with residents and tourists taking advantage of the late summer weather, and hundreds of world leaders and dignitaries arriving this week for the United Nations General Assembly.
United Nations spokesman Stephane Dujarric told VOA that officials assess security needs inside the U.N. complex on a daily basis.
"Outside of the gates, we are in the hands of the host country, at the federal and local levels,” he said. “We appreciate their work and, no doubt, they are doing their utmost to keep everyone safe,” he added.
Chelsea resident Michelle Katz told VOA she was in bed when something sounded "like a bomb exploding or a truck driving into a building."
Two minutes later, "there were endless sirens," Katz told VOA.
Another witness, Denise Coles, said the bomb went off as she was pulling into a parking place.
"I turned the car off and that's when we heard the explosion. It was like you could feel it inside of you. I looked down the block and I saw the smoke coming out."
"We're fortunate that this didn't happen during the week, like a work day, a Monday or Tuesday," said Steven Faria, who works at a nearby veterans' hospital. "With all the people that work in this area, I think the casualties would have been twice as many people."
City and federal law enforcement also are investigating an unexploded device found just a few blocks from the Chelsea bomb. Police say it looks like a pressure cooker attached to a cell phone and wires in a plastic bag.
The New York blast came hours after a pipe bomb exploded in a trash bin in a New Jersey beach town Saturday 135 kilometers away, forcing the cancellation of a charity foot race involving thousands of runners. No one was hurt and De Blasio and Cuomo have said there is no evidence linking the two incidents.
VOA reporters Margaret Besheer, Esha Sarai, Ramon Taylor, Steve Herman, and Celia Mendoza contributed to this report from New York.