The FBI has searched a mail facility in Florida, as investigators try to find out who is behind a series of crude explosive packages targeting several prominent Democrats.
Devices have been sent to eight high-profile people, including ex-President Barack Obama and actor Robert De Niro.
US officials say some of the packages appear to have been sent from a postal centre in north-western Miami.
The FBI has not yet released any details of their investigation.
Similar packages have also been sent to former Vice-President Joe Biden and former Secretary of State Hillary Clinton, among others.
Security camera footage at the postal facility in Opa-locka, Miami, is being scrutinised. A bomb squad and canine unit had accompanied federal officials to the site, police from Miami-Dade County said.
On Thursday evening, the Time Warner Center in New York was evacuated after reports of a suspicious package, but it was later declared a false alarm. The building is home to news network CNN, which received one of the devices by post the previous day.
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.
Anthony May, a former government bomb expert, told NPR the packages contained only one set of batteries - not enough to trigger the explosive.
Earlier, Mr Sweeney confirmed that white powder found inside the package sent to CNN "did not present a biological threat".
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".
"This has to be taken with the utmost seriousness," he said. "As far as a hoax device? We're not treating it that way."
Mr O'Neill added that authorities "do not know at this time" whether there were additional devices in the mail.
On Thursday morning, the FBI said Mr Biden had received two suspect packages and a similar package was found at a New York restaurant owned by Mr De Niro.
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.
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
- Former CIA Director John Brennan, care of CNN
- Former Attorney General Eric Holder
- California Democratic Congresswoman Maxine Waters
- Mr De Niro
None of the devices went off.
What happened with De Niro?
A suspected explosive device was found at a building in Manhattan early on Thursday, local time. It is home to Mr De Niro's production company and the Tribeca Grill restaurant.
New York City Mayor Bill de Blasio praised the "really quick-witted work of a security guard" who alerted authorities.
According to the Associated Press, the security guard was off work on Thursday and had seen an image in a news report of the packages others had received.
That individual then recalled spotting something similar in the building's mailroom and immediately called authorities who removed the device at about 06:30 local time (10:30 GMT), AP reports.
CNN's New York office was evacuated on Wednesday morning after the package addressed to Mr Brennan was found in its mailroom.
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.
On Wednesday, President Trump responded to the unfolding news of the suspect packages by calling for more civility in public life, and urging 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.