A 56-year-old man has been arrested in Florida in connection with a mail-bombing campaign aimed at critics of 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, 56, 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. Court records show he filed for bankruptcy in 2012.
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 former intelligence chief James Clapper was discovered in New York City.