A 56-year-old man has been arrested in Florida in connection with a mail-bombing campaign aimed at critics of President Trump.
US officials named the man as Cesar Sayoc, saying that more arrests could follow.
Twelve packages have been sent to figures such as former president Barack Obama and actor Robert de Niro in recent days.
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.
The FBI has reportedly been searching a mail facility in Florida for clues about the packages.
Media reports say Mr Sayoc was arrested at an auto 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 a criminal history.
Mr Trump has commented several times on the incidents, but in his latest tweet he suggested the incidents, which he described as “‘Bomb’ stuff”, were slowing Republican “momentum” in early voting.
The BBC’s Nick Bryant in Washington says he appears to be suggesting that the bombing campaign is a hoax.
But 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 former intelligence chief James 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.
- Mr 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)
- Mr De Niro
None of the devices went off.
Why is this so political?
Mr Trump’s initial response to the unfolding news of the suspect packages was to call for more civility in public life, and urge politicians to stop treating their opponents as “morally defective”.
His critics called his remarks hypocritical, and former CIA Director John Brennan tweeted to him on Thursday: “Stop blaming others. Look in the mirror. Your inflammatory rhetoric, insults, lies, & encouragement of physical violence are disgraceful.”
Mr Trump on Friday threw back the accusations in a tweet, complaining that his opponents criticised him “at will” and yet cried foul when he attacked them back.
Conservatives say Democrats are to blame for the state of political discourse in the US.
They say Democrats, including those who were targeted in the bomb scare, have encouraged “angry mob” behaviour.
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”.