A 56-year-old man has been arrested in Florida in connection with a mail-bombing campaign aimed at critics of US President Donald Trump.
US officials named the man as Cesar Sayoc, saying that more arrests could follow.
Mr Trump said the acts were “despicable and have no place in our country”, and promised “swift and certain justice”.
Twelve items have been sent in recent days to figures including ex-President Barack Obama and actor Robert de Niro.
The latest two were found in Florida and New York City on Friday.
The incidents come less than two weeks before the US mid-term elections, with politics highly polarised.
Media reports say Mr Sayoc was arrested at an vehicle parts shop in the city of Plantation, Florida.
US officials say he is the principal suspect, and that DNA evidence was used to track him down.
He reportedly has ties to New York and is a registered Republican. Public records show he has a long criminal history and on one occasion was accused of threatening to use a bomb.
Court records show he filed for bankruptcy in 2012.
US TV broadcast live images of a van, said to belong to Mr Sayoc, being loaded on to a trailer in Plantation and towed away for examination.
What did Mr Trump say about the bombs?
Speaking at an event at the White House, Mr Trump praised law enforcement for the quick apprehension of the subject, describing the search as looking for a “needle in a haystack”.
“These terrorising acts are despicable and have no place in our country,” he said.
“We will prosecute… them to the fullest extent of the law. We will never allow political violence to take root in our country.”
The comments were in stark contrast to Mr Trump’s tweet earlier in the day, when he suggested the incidents, which he described as “‘Bomb’ stuff”, were slowing Republican “momentum” in early voting.
Former intelligence chief James Clapper, one of the recipients of Friday’s packages, told CNN: “This is definitely domestic terrorism, no question in my mind.”
He said that anyone who had been a critic of President Trump needed to be on the alert and take extra precautions.
How did the bomb threat unfold?
The series of bomb alerts began on Monday, when a suspected device was found in the post box of billionaire businessman George Soros, a major Democratic Party donor.
The latest came on Friday, when a package addressed to top Democratic Senator Cory Booker was found in Florida and a second to Mr Clapper was discovered in New York City.
Before that, a total of 10 devices were sent to the following eight individuals, according to the FBI:
- George Soros
- Former Secretary of State Hillary Clinton
- Former President Barack Obama
- Former Vice-President Joe Biden (two devices)
- Former CIA Director John Brennan, care of CNN
- Former Attorney General Eric Holder
- California Democratic Congresswoman Maxine Waters (two devices)
- Robert De Niro
None of the devices went off.
What was inside the packages?
FBI Assistant Director William Sweeney said thorough examinations of all the packages were under way at the FBI laboratory in Quantico, Virginia, near Washington DC.
Several of the packages appear to have contained pipe bombs, according to the FBI.
CNN quotes investigators as saying they were functional but unstable, meaning they could be set off merely by handling. They have timers easily bought at retail outlets.
But experts speaking to several US media outlets have cast doubt on their effectiveness after seeing X-ray images.
New York Police Commissioner James O’Neill could not confirm whether all the devices were intended to explode, but he said officials “are treating them as suspected explosive devices”.
