Two suspicious packages have been found in the US, addressed to a top Democrat and a former senior intelligence official.
One item addressed to Senator Cory Booker was found at or near a Florida mail facility.
A second was at a New York post office, addressed to ex-national intelligence chief James Clapper via CNN.
The news came after 10 mail bombs were sent to prominent critics of US President Donald Trump.
Ex-President Barack Obama and actor Robert de Niro were among those targeted.
Security camera footage at the Florida postal facility in Opa-locka, Miami, is being scrutinised by the FBI. A bomb squad and canine unit had accompanied federal officials to the site, police from Miami-Dade County said.
Meanwhile New York police described the Manhattan package as one that "closely resembled others sent across the country in recent days".
They said bomb squad personnel were on the scene, later adding that the package had been removed.
Mr Clapper responded immediately to the news that he had been targeted, saying on CNN: "This is definitely domestic terrorism, no question in my mind.
"There is some question about whether these things are functional or not - we probably have to assume they actually are," he added, saying the incidents were not going to silence critics of the Trump administration.
But he said that anyone who had in any way publicly 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.
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?
Every person who has has been sent a package is known to be a critic of President Donald Trump.
The attempted bombings also come just under two weeks before the mid-term elections, with US politics highly polarised.
Mr Trump has commented several times on the mail bombs, but in his latest tweet he suggested the incidents, which he described as "'Bomb' stuff", were slowing Republican "momentum" in early voting.
Republicans are doing so well in early voting, and at the polls, and now this “Bomb” stuff happens and the momentum greatly slows - news not talking politics. Very unfortunate, what is going on. Republicans, go out and vote!— Donald J. Trump (@realDonaldTrump) October 26, 2018
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."
Stop blaming others. Look in the mirror. Your inflammatory rhetoric, insults, lies, & encouragement of physical violence are disgraceful. Clean up your act....try to act Presidential. The American people deserve much better. BTW, your critics will not be intimidated into silence. https://t.co/cS5qNiuU7o— John O. Brennan (@JohnBrennan) October 25, 2018
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.
Funny how lowly rated CNN, and others, can criticize me at will, even blaming me for the current spate of Bombs and ridiculously comparing this to September 11th and the Oklahoma City bombing, yet when I criticize them they go wild and scream, “it’s just not Presidential!”— Donald J. Trump (@realDonaldTrump) October 26, 2018
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".