City, state and federal lawmen on Sunday intensified an investigation into the explosion that injured 29 people in Manhattan’s Chelsea neighborhood, while Mayor Bill de Blasio declared that the city won’t be intimidated by the violent act.

The explosion, described by one neighbor as "deafening," happened at about 8:30 p.m. outside the Associated Blind Housing facility at 135 W. 23rd Street. The facility provides housing, training and other services for the blind.

Police later found a second explosive device nearby on 27th Street.

The mayor said security measures will be increased at key locations around the city in the wake of the bombing. He noted that law enforcement will be particularly focused at the United Nations building in light of the scheduled session this week of the General Assembly.

“You will see a very substantial New York Police Department presence this week,” De Blasio said. “The biggest ever.

The mayor asked citizens to be watchful and report any suspicious activity to local authorities.

“Be vigilant,” he said.

Hundreds of people were seen fleeing down the block Saturday evening after the blast, as police cordoned off the area.

"It was really loud, it hurt my eardrums. My 10-year-old boy was sat in the back seat of the car, and the explosion blew the back window out," said Tsi Tsi Mallett, who was in a car driving along 23rd Street when the explosion took place. Her son was not injured.

Neha Jain, 24, who lives in the neighborhood, said she was sitting in her room watching a movie when she suddenly heard a huge boom and everything shook.

"Pictures on my wall fell, the window curtain came flying as if there was a big gush of wind. Then we could smell smoke. Went downstairs to see what happened and firemen immediately told us to go back."

New York City Police issued a bulletin advising motorists in the area that they should "expect extensive traffic delays and emergency personnel in the area of 23rd Street and 7th Avenue" due to police activity there and asking the public to avoid the area.