A man has been charged over a series of pipe bombs sent to prominent critics of Donald Trump after his fingerprint was found on one of the devices.
Cesar Sayoc, from Aventura, Florida, was arrested in Miami after 13 explosive devices were sent to several high-profile figures, including Hillary Clinton, Barack Obama and Robert De Niro.
US justice department officials said the bombs were "not hoax devices" and warned there could be more still undiscovered.
Sayoc, 56, was identified after officers found a fingerprint on one of the packages, FBI director Christopher Wray said.
Speaking at a news conference, US attorney general Jeff Sessions said the suspect could face up to 48 years in prison if found guilty but reiterated that he was "innocent until proven guilty".
He said the Sayoc, who "appears to be a partisan", was facing five federal charges which included illegally mailing explosives and the interstate transportation of explosives.
The charges also include making threats against former presidents, threatening interstate commerce and assaulting federal officers.
Sending a warning to others, Mr Sessions said: "We will find you and we will prosecute you to the full extent of the law."
Mr Wray said the devices consisted of six inches of PVC pipe, a small clock, a battery, and "energetic material" that could give off "heat energy" in response to any heat or friction.
He said there could be more suspicious packages and urged people to remain vigilant.
Mr Wray said it was too early to discuss any possible motives and that officials were still investigating whether other people were involved in sending the packages.
According to court records, Sayoc – an amateur bodybuilder – has a string of arrests for theft and illegal steroids possession. In 2002, he was charged with making a bomb threat.
His social media accounts contain numerous posts denouncing Democrats and praising Mr Trump.
His arrest came after police intercepted suspicious packages sent to Democrats Kamala Harris and Cory Booker, and the former director of national intelligence James Clapper.
Mr Sessions said: "Let this be a lesson to anyone regardless of their political beliefs that we will bring the full force of law against anyone who attempts to use threats, intimidation and outright violence to further an agenda."
In Florida, officers were seen examining and seizing a white van which had several stickers on its windows, including American flags, Republican logos, a photo of Mr Trump and one which read: "CNN sucks."
Following the arrest, speaking at an event for young black conservatives, Mr Trump said "we must never allow political violence take root in America", saying the country "must unify".
Earlier in the day the US president complained on Twitter that this "bomb stuff" had slowed momentum for the Republicans ahead of the US midterm elections, which are less than two weeks away.
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"Republicans are doing so well in early voting, and at the polls, and now this 'Bomb' stuff happens and the momentum greatly slows – news not talking politics," he wrote. "Very unfortunate, what is going on. Republicans, go out and vote!"
Others sent suspicious packages include former vice president Joe Biden, billionaire philanthropist George Soros, and former CIA head John Brennan.