FBI agents have searched a mail processing centre in Florida as they race to find out who sent 10 pipe bombs to high-profile Democrats and critics of Donald Trump.
A police bomb squad and canine units joined the federal investigators as they scoured the sprawling warehouse outside of Miami, where officials believe several of the suspicious packages were processed.
William Sweeney, an assistant director of the FBI, has warned it is possible that further pipe bombs could be sent – with actor Robert De Niro and former vice president Joe Biden the latest targets.
The parcel bombs are being regarded as “live” explosives rather than a hoax, and they are currently being examined by technicians at the FBI’s forensic lab in Virginia.
None of the devices detonated and no one has been hurt – but nonetheless, officials have described the attempted bombings as an act of terrorism.
Bomb experts and security analysts have said the basic construction of the bombs makes it seem like they were designed to sow fear rather than kill people.
Although the devices did contain timers and batteries, they were not rigged like booby-trapped bombs that detonate as soon as they are opened.
All of the recipients are linked – either as a political rival to the US president or one of his critics.
Mr Trump has long questioned Barack Obama’s US citizenship, and described Hillary Clinton as “crooked” during the 2016 presidential campaign.
The president revoked the security clearance of former CIA director John Brennan after he attacked Mr Trump’s performance at a Russia summit as “nothing short of treasonous”.
Meanwhile, Robert De Niro received a loud ovation back in June when he hurled an obscenity at Mr Trump during an awards ceremony – and apologised to Canadians for the “idiotic behaviour of my president”.
Mr Trump’s opponents have claimed that his inflammatory rhetoric against Democrats and the media has contributed to the nationwide scare.
But on Twitter, the US president disagreed, writing: “A very big part of the Anger we see today in our society is caused by the purposely false and inaccurate reporting of the Mainstream Media that I refer to as Fake News. It has gotten so bad and hateful that it is beyond description. Mainstream Media must clean up its act, FAST!”
His remarks led to a fiery response from one of those who had a pipe bomb addressed to them.
On Twitter, John Brennan retorted: “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 four-day saga has ratcheted up tensions ahead of next week’s midterm elections – and sparked a series of security alerts.