The bomb detonated in New York on Saturday, and another device discovered nearby, were both shrapnel-filled pressure cookers – similar to the bombs used at the 2013 Boston marathon, reports say.
Mentioning officials, different reports said both had Christmas lights and mobile phones as makeshift detonators.
The blast, in the Chelsea area of Manhattan, injured 29 people.
The FBI said it stopped a “vehicle of interest” in Brooklyn on Sunday but made no arrests.
Five people were taken into custody for questioning, officials told US media. But a spokeswoman said no one had been charged and the investigation was continuing.
The Manhattan blast followed a pipe bomb explosion on the route of a charity race.
Investigators will also be examining a suspicious device found near a train station in Elizabeth, New Jersey.
Elizabeth Mayor Christian Bollwage said a bomb squad had discovered the device including cables and conduits could be a live bomb.
Reporters for two US news outlets said a loud explosion was heard in the place early on Monday.
New York Governor Andrew Cuomo said a second device discovered over the weekend in the city appeared to be “similar in design” to the exploded device.
It was discovered four blocks in the site of the blast and was removed and later destroyed in a controlled explosion. No one was hurt.
Previously on Sunday, New York Mayor Mayor Bill de Blasio said: “Was it a political motivation? A personal motivation? We do not know.”
“We know there was a bombing. We understand it’s a very serious incident. But we have a lot more work to do to be able to say what kind of motivation was behind this.
“All conceivable theories of what’s happened here and how it connects will be looked at but we’ve no particular evidence only at that point in time.”
“Whoever set these bombs – we are going to find them and they are going to be brought to justice.”
Some 1,000 extra security personnel are being deployed to New York ‘s transport hubs, authorities said.
The Chelsea explosion occurred at about 21:00 (01:00 GMT on Sunday) on West 23rd St. Windows were blown out by the force of the blast and could be heard several blocks away.
Some reports said the bomb went off in a black metal construction toolbox, others that it was in a rubbish bin.
Chelsea is one of the most fashionable districts of Manhattan and its bars and restaurants are usually packed at the weekend.
Mr de Blasio said there would be a “larger than ever” police presence in Nyc in the coming week.
The weekend’s attacks in Minnesota, New Jersey and Ny have put national security directly back in the center of the US presidential election.
A similar scenario unfolded in June following the Orlando club shootings. That was an opportunity for Donald Trump – the “change” nominee who touts his hard line on security problems – to make his claim… and he blew it. Mrs Clinton’s lead grew as Americans soured on Mr Trump’s bellicose response.
This time Mr Trump has been more calculated, offering condolences that were only . He did call the Nyc event a “blasting” before official confirmation and said the US needed to “get tough”, but that has been hardly comparable to his jarring opinions on Orlando.
Mrs Clinton cautioned against early conclusions – possibly hoping her challenger would overreact.
Under extreme pressure, both candidates are with the race the first TV debate only over a week away and tight once again to display their leadership qualities. Saturday’s “nightmare scenario” was another evaluation – and it likely will not be the last.