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. He faces five charges including mailing explosives and threatening ex-presidents.
Mr Trump said the acts were "despicable and have no place in our country".
Fourteen items have been sent in recent days to figures including ex-President Barack Obama and actor Robert de Niro.
Two were found in Florida and New York City on Friday morning.
Later, two more were discovered in California. Billionaire and Democrat donor Tom Steyer said that a package sent to him had been intercepted at a mail facility in Burlingame, and another addressed to Democrat Senator Kamala Harris was reported in Sacramento.
The incidents come less than two weeks before the US mid-term elections, with politics highly polarised.
How did Mr Trump react?
The president praised law enforcement for the quick arrest of the suspect, 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.
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.
"Bomb stuff"? The President of the United States appears here to be fuelling the conspiracy theories that this is a liberal hoax.... https://t.co/NfKdNVs7uj— Nick Bryant (@NickBryantNY) October 26, 2018
But Mr Trump returned to the theme later, accusing US media of exploiting the latest case.
"The media's constant, unfair coverage, deep hostility and negative attacks... only serve to drive people apart and to undermine healthy debate," he said at a rally in North Carolina.
"We have seen an effort by the media in recent hours to use the sinister actions of one individual to score political points against me and the Republican party."
US media reports suggest Mr Sayoc is a registered Republican who attended some of Mr Trump's rallies in 2016 and 2017. However, the president rejected any suggestion that his rhetoric had contributed to the attacks.
"I heard he was a person that preferred me over others. There's no blame, there's no anything," Mr Trump said.
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.
"I'm not suggesting a direct cause-and-effect relationship between anything he's said or done and the distribution of these explosives. But I do think he bears some responsibility for the coarseness of civility of the dialogue in this country," he added.
How Cesar Sayoc was caught?
He was at a vehicle parts shop in the city of Plantation, Florida.
FBI Director Christopher Wray revealed that he was detained after his fingerprint was allegedly found on one of the packages.
Officials also said DNA and mobile phone data were used to track the suspect down.
The Department of Justice said he faced up to 48 years in jail.
"We will not tolerate such lawlessness, especially political violence," US Attorney General Jeff Sessions said at a news conference.
"Let this be a lesson to anyone, regardless of their political beliefs, that we will use the full force of the law against you."
What do we know about Cesar Sayoc?
Law enforcement agencies said Mr Sayoc lives in Aventura, Florida.
In 2002, he was arrested for making a bomb threat in Miami-Dade County, and received one year of probation for the charge.
Mr Sayoc has a criminal record dating back to 1991 in Broward County, according to clerk records. He was arrested, aged 29, on a theft charge. He has also faced charges of fraud and battery.
Court records show Mr Sayoc filed for bankruptcy in 2012 while he was living with his mother. A handwritten note in his bankruptcy report reads: "Lives w/mom. Has no furniture."
In 1980, he spent three semesters as a student at Brevard College in North Carolina, a university spokeswoman told BBC News. He did not graduate, the spokeswoman added.
Following his arrest, US TV broadcast live images of a white van, said to belong to Mr Sayoc, being loaded on to a trailer in Plantation and towed away for examination.
The van's windows were covered in images. One piece of artwork depicted President Trump standing on a tank and another showed Hillary Clinton with a bullseye superimposed on her face.
Twitter and Facebook accounts in the name of Cesar Altieri and Cesar Altieri Randazzo respectively, believed to be used by the suspect, have been taken down.
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.
A total of 13 devices were sent to the following 11 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
- Democrat Senator Cory Booker
- Former director of national intelligence James Clapper
- Democrat Senator Kamala Harris
The package sent to Mr Steyer was not mentioned on the complaint against Mr Sayoc.
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".
Later FBI Director Wray said that, though they were still being examined, "these are not hoax devices".
He said it was possible there were more undiscovered packages.
Some of the packages included photos of the intended targets with red Xs drawn through them, investigators said.