The bomb detonated on Saturday in New York, and another device found nearby, were both shrapnel-filled pressure cookers – similar to the bombs used at the 2013 Boston marathon, reports say.
Citing officials, separate reports said both had mobile phones and Christmas lights 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 in New Jersey on Sunday.
Investigators are also examining a suspicious device found near a train station in Elizabeth, New Jersey.
Elizabeth Mayor Christian Bollwage said a bomb squad had determined the device containing wires and pipes could be a live bomb.
Reporters for two US news outlets said a loud explosion was heard in the area early on Monday.
- How events unfolded
- ‘People running for their lives’
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 found four blocks from the site of the blast and was removed and later destroyed in a controlled explosion. No-one was hurt.
Earlier 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 know 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 possible theories of what’s happened here and how it connects will be looked at but we have no specific evidence at this point in time.”
Mr Cuomo said: “Whoever placed these bombs – we will find them and they will be brought to justice.”
Some 1,000 extra security personnel are being deployed to New York’s transport hubs, police said.
Windows blown out
The Chelsea explosion occurred at about 21:00 (01:00 GMT on Sunday) on West 23rd St. The force of the blast blew out windows 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 among the most fashionable districts of Manhattan and its bars and restaurants are usually crowded at the weekend.
Mr de Blasio said there would be a “bigger than ever” police presence in New York in the coming week.
On Tuesday, President Obama and other world leaders are due to attend the UN General Assembly in New York.
How the election candidates reacted – BBC’s Anthony Zurcher
Governor Andrew Cuomo called it a politician’s “nightmare scenario” – a late-night phone call with news of a crisis. The weekend’s attacks in New York, New Jersey and Minnesota have put national security squarely back in the centre of the US presidential election.
A similar scenario unfolded in June following the Orlando nightclub shootings. That was an opportunity for Donald Trump – the “change” candidate who touts his hard line on security issues – to make his case… 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 measured, offering only condolences on Twitter. He did call the New York incident a “bombing” before official confirmation and said the US needed to “get tough”, but that was hardly comparable to his jarring comments on Orlando.
Meanwhile, Mrs Clinton cautioned against premature conclusions – perhaps hoping her opponent would again overreact.
With the race tight once again and the first TV debate just over a week away, both candidates are under intense pressure to display their leadership qualities. Saturday’s “nightmare scenario” was another test – and it probably won’t be the last.
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Source: BBC