Former US president Barack Obama and former secretary of state Hillary Clinton were among the targets of pipe bombs delivered to several high-profile Democrats and CNN in what New York officials have called an act of terrorism.
At least eight parcels were intercepted before reaching any intended recipients, including Obama’s attorney-general, Eric Holder, former CIA director John Brennan and prominent Democratic Party donor George Soros.
Two packages were sent to California congresswoman Maxine Waters, the FBI said.
Investigators were trying to track down a suspicious package they believe was addressed to Obama’s vice-president, Joe Biden, a federal official told Reuters late on Wednesday.
None of the eight packages detonated and nobody was hurt. There was no immediate claim of responsibility.
But news of the threats heightened tensions in a nation deeply polarised before elections on November 6 that will decide whether Democrats take control of one or both houses of Congress from Republicans and deny President Donald Trump the majority his party now holds in both.
Trump told a rally in Wisconsin his government would conduct “an aggressive investigation”.
“Any acts or threats of political violence are an attack on our democracy itself,” Trump said.
“We want all sides to come together in peace and harmony.”
But he said the media has a responsibility “to stop the endless hostility and constant negative and oftentimes false attacks and stories”.
The first package, which turned up on Monday, was addressed to Soros, the billionaire financier and frequent target of right-wing conspiracy theories.
The parcel intended for Holder ended up rerouted to the return address printed on all the packages – the Florida office of US Representative Debbie Wasserman Schultz, who formerly chaired the Democratic National Committee.
The FBI said late on Wednesday it had confirmed two additional packages, similar in appearance to the others, were addressed to US Representative Waters, a Democratic congresswoman and outspoken Trump critic.
An earlier FBI statement said the other packages consisted of a manila envelope with a bubble-wrap interior containing “potentially destructive devices”.
Other officials said the devices contained in the envelopes were similar to one found in the mailbox of the Soros home and later detonated by police.
At least one bomb was packed with shards of glass, one federal source said.
The US Secret Service intercepted packages addressed to Obama at his Washington home and to Clinton, the 2016 Democratic nominee for president, at the New York state home she shares with former president Bill Clinton.
The CNN bureau in New York also received a package addressed to Brennan, who has appeared as a CNN analyst, leading police to evacuate the Time Warner building.
The package to Clinton was found late on Tuesday and the one addressed to Obama was recovered early on Wednesday, both during routine off-site mail screenings, the Secret Service said.
The package sent to CNN also contained an envelope of white powder that experts were analysing.
“This clearly is an act of terror attempting to undermine our free press and leaders of this country through acts of violence,” New York Mayor Bill de Blasio said.
All the targets are frequently maligned by right-wing critics. Trump has repeatedly criticised CNN as “fake news” and disparaged the mainstream news media as an “enemy of the people.”
“There is a total and complete lack of understanding at the White House about the seriousness of their continued attacks on the media,” CNN President Jeff Zucker said in a statement read on CNN.
Senate Democratic Leader Chuck Schumer and House of Representatives top Democrat Nancy Pelosi said Trump’s call for unity rang “hollow” because of his past statements condoning acts of violence.