The US has seen a number of incidents were explosive or toxic devices have been sent to public figures through the mail.
A number of viable bombs packed with glass have been posted this week to senior US politicians and Democratic Party supporters.
Here are some of the most notable previous incidents:
:: The 1919 anarchist bombings
Letter bombs were deployed just under a century ago sent by radical anarchists. A round were sent to judges, politicians, law enforcement officials and others across eight cities.
A US senator was among those targeted. He survived, but his nurse had her hands blown off by the bomb.
The next day, 12 more explosives were sent in packages to political and business leaders – including John Rockefeller.
:: The Unabomber
For nearly 20 years, an anonymous figure known only as "the Unabomber" became the most wanted figure in America.
Ted Kaczynski was given the nickname by the FBI for his string of attacks on a university and airline. His devices killed three people and injured 24 more.
The taskforce that tried to track him down hired 150 full-time investigators. His victims were chosen randomly from library research.
Kaczynski was only discovered when he released a 35,000-word essay, later carried in two national newspapers.
His brother came forward and set police on track to finding him in a small, primitive cabin the two siblings had constructed themselves.
:: 2001 Amerithrax attacks
A week after the 9/11 terror attack, letters laced with anthrax were posted out.
Five people were killed an 11 sickened in what became the worst biological attack in US history.
The investigation was codenamed "Amerithrax".
One senator was sent a note reading: "You cannot stop us. We have this anthrax. You die now. Are you afraid? Death to America. Death to Israel. Allah is great."
The FBI said it was about to name Dr Bruce Ivins as the culprit – but he killed himself before they did.
:: 2018 Austin bombing campaign
A 23-year-old with no criminal history blew himself up in his car as police closed in after a spate of deadly letter bombs.
Mark Conditt's first three devices were hidden in packages that were not posted but placed on people's doorsteps.
A fourth was set off using a tripwire tied to a "caution – children at play" sign. A fifth was posted from a FedEx shop.
He was identified from the CCTV footage.
:: 2018 toxic Trump letter
Last Wednesday, the justice department announced it was charging a man with sending the toxin ricin to Donald Trump in a letter.
It included the words "Jack and the Missile Bean Stock Powder" and contained castor bean material, the plant ricin comes from.
William Clyde Allen III is also charged with five other charges of "mailing threatening communications to an officer or an employee of the US".
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