As many as 1,000 additional security forces were patrolling the streets of New York City on Sunday after a powerful blast set off in a garbage bin injured 29 people in Manhattan's hip neighbourhood of Chelsea the night before.
New York Governor Andrew Cuomo said the added state police and members of the US National Guard were meant to ensure that the day-to-day activity in the city went uninterrupted and that residents felt secure.
"At this time there is no evidence of an international terrorism connection," the governor told journalists in New York. Earlier Sunday, Mayor Bill de Blasio had called the explosion an "intentional act" but also rejected a terrorism connection.
Cuomo said the explosive device appeared to be a pressure cooker but that officials were waiting for an analysis from the Federal Bureau of Investigation.
"We will find whoever did this or whatever group did this, and they will be brought to justice, period," Cuomo said, adding that investigators were reviewing surveillance videos from the explosion area.
The explosion, which occurred late Saturday outside a building on West 23rd Street, comes less than one week after the city commemorated the 15th anniversary of the terrorist attacks of September 11, 2001.
After the explosion, a second suspicious device was found several blocks north on West 27th Street, and a bomb disposal unit had removed it from the area for analysis, New York police said.
The blast blew out nearby windows and hurled debris across the street, images posted on social media and surveillance video showed.
The White House said President Barack Obama was receiving updates on the ongoing investigation.
The New York City incident occurred hours after a bomb hidden in a garbage container exploded Saturday in the neighbouring state of New Jersey along the planned route of a charity run sponsored by the US Marines, causing no injuries.
De Blasio said there were currently no indications of a connection to the earlier blast in New Jersey.