A manhunt is under way for the culprit who has sent mail bombs to high-profile Democrats and CNN, just under two weeks away from the US mid-term elections.
Barack Obama, Joe Biden, Hillary Clinton and Robert de Niro are among those who have been sent suspicious packages, none of which detonated.
What are the clues so far?
Glass-filled pipe bombs
Detectives say the device sent to Joe Biden is similar to the glass-filled pipe bombs addressed to Barack Obama, Hillary Clinton and other Democrats.
An image of the device sent to CNN shows a small, rudimentary explosive wrapped in black electrical tape and a digital timer attached to the centre.
Because the bombs were intercepted before they exploded, forensics experts will be able to carefully disassemble the devices and examine the components.
Officials have confirmed that the devices found so far all have matching features – with US media reporting that they were all made from PVC pipe with a digital timer attached to set off the detonator.
X-rays also showed there were pieces of shrapnel inside the pipes.
Bomb technicians have since confirmed that the devices have the necessary components to cause an explosion.
BREAKING: AP source: Package addressed to Rep. Maxine Waters found at Los Angeles mail facility; appears similar to NY, DC bombs.
— The Associated Press (@AP) October 25, 2018
Bomb packages similar in appearance
All of the bombs were packaged in large manila envelopes with bubble wrap.
The office of congresswoman Debbie Wasserman Schultz, the former Democratic National Committee chairwoman, is listed as the return address.
The FBI says it is “possible that additional packages were mailed to other locations.”
It added: “The packages are similar in appearance and contain potentially destructive devices.”
Bomb packages carefully taped
Each of the packages are carefully taped, with the culprit likely to have used gloves to not leave behind fingerprints or DNA.
Investigators will scrutinise the postal system or courier service used to deliver the bombs.
The US Postal Service operates a sophisticated imaging system that photographs the outside of each piece of mail processed across the country and can be used to determine the specific location of where it was sent.
The packaging materials themselves will also be closely examined.
“It will be a treasure trove of forensic evidence,” said Anthony Roman, a private security and investigations consultant.
Breaking: Pic of suspected bomb package sent to Former Vice President Joe Biden in postal facility in Delaware. It appears to not have been delivered because of “postage due”as at least one reason along with wrong address. Same return address label as others sent. @FOX29philly pic.twitter.com/mB55K5Kn9X
— Steve Keeley (@KeeleyFox29) October 25, 2018
“As human beings, we are filtering off our DNA everywhere we walk, everywhere we sit.”
Targets frequently criticised by right-wing politicians and pundits
All of the targets are regularly criticised by right-wing politicians and pundits, including Mr Trump himself.
In a statement, two senior Democrats said: “Time and time again, the president has condoned physical violence and divided Americans with his words and his actions.”