This is the latest development after authorities arrested a Florida man and charged him with five federal crimes.
ADVERTISEMENTA 56-year-old man has been arrested in Florida in connection with a mail-bombing campaign aimed at critics of US President Donald Trump.
Who is Cesar Sayoc?
US officials named the man as Cesar Sayoc.
FBI agents arrested Sayoc in Plantation, Florida and also hauled away a white van belonging to him that was plastered with pro-Trump stickers, the slogan "CNN SUCKS" and images of Democratic figures with red crosshairs over their faces.
Sayoc has worked as a male stripper, a professional wrestler, a shop manager and a club promoter, according to Reuters.
He used his social media accounts to air political grievances, with liberal billionaire George Soros, former president Barack Obama and former secretary of state Hillary Clinton among those he targeted on Twitter.
Records show that he was a registered Republican.
Over the years, he was arrested for domestic violence, theft and fraud.
He delivered pizzas at the New River Pizza & Fresh Kitchen in Fort Lauderdale. His former boss, Debra Gureghian, described him as "racist" and "anti-everything".
But, she said, he was a good employee who could always be counted on.
"Just because he didn`t like me, or someone that is black, or gay or transgender or Jewish, I couldn`t fire him. He did his job. He bagged up everything. He never forgot anything. I`'m calling drivers saying you forgot something, he was attentive. He knew what to grab. He didn`t skip a beat."
What are the details of the arrest?
Sayoc faces five charges including threats against former presidents and could be imprisoned for up to 48 years if found guilty, US Attorney General Jeff Sessions said.
"We will not tolerate such lawlessness, especially political violence," he said.
The investigation into this week's wave of suspicious packages focused on southern Florida and a mail sorting facility in the area.
FBI agents used fingerprint and DNA evidence to identify the suspect. FBI Director Christopher Wray told a news conference that fingerprints on a package sent to Representative Maxine Waters belonged to Sayoc.
Authorities are still investigating whether other people were involved and did not rule out the possibility of further arrests or more explosive devices in the mail.
Trump's reaction
Announcing the arrest to a cheering audience at the White House, Trump said such "terrorising acts" were despicable and had no place in the United States.
"We must never allow political violence to take root in America - cannot let it happen," Trump said. "And I'm committed to doing everything in my power as president to stop it and to stop it now."
Hours after a federal law enforcement official said the investigation's focus on Florida had intensified, police closed roads around the parking lot of an AutoZone store in Plantation where Sayoc was arrested, and helicopters flew overhead. Investigators covered Sayoc's van with a blue tarp before removing it on a truck.
ADVERTISEMENTTrump's critics charged that his inflammatory rhetoric against Democrats and the press created a climate for politically motivated violence.
Tensions are running high ahead of the November 6 elections that could shift the balance of power in Congress, which is currently controlled by Trump's fellow Republicans.
"If we don't stop this political mania, this fervour, rancour, hatred, you'll see this again and again and again," New York Governor Andrew Cuomo told MSNBC. "We have to get to the genesis, and the genesis is an overheated, vitriolic political division in this country and it starts with the leaders, and it starts with the president."
Trump's supporters accused Democrats of unfairly suggesting the president was to blame for the bomb scare.
After first calling for unity at the White House event, Trump lamented attacks against him and again pointed at the media.
ADVERTISEMENT"I get attacked all the time ... I can do the greatest thing for our country, and on the networks and on different things it will show bad," he told the crowd, acknowledging an attendee who shouted "fake news."
How did the bomb threat unfold?
Fourteen items have been sent in recent days to figures including actor Robert de Niro and former president Barack Obama.
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 series of bomb alerts began earlier this week when a suspected device was found in the post box of billionaire businessman George Soros, a major Democratic Party donor.
ADVERTISEMENTHow many packages have been sent so far?
Packages intercepted by authorities were addressed to:
Billionaire philanthropist and financier 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
US Congresswoman Debbie Wasserman Schultz and former Attorney General Eric Holder
California Democratic Congresswoman Maxine Waters (two devices)
Actor Robert De Niro
Democrat Senator Cory Booker
Former director of national intelligence James Clapper
Democrat Senator Kamala Harris
California billionaire Thomas Steyer