Hollywood megastar Robert De Niro and former US vice president Joe Biden were the latest targets in a spree of 10 devices and pipe bombs sent to opponents of Donald Trump but a nationwide manhunt revealed no major public leads Thursday.
Hundreds of agents from the FBI, US Secret Service and dozens of other agencies are searching round the clock for the culprit or culprits behind the 10 suspect packages that politicians on all sides have branded "terrorism."
Since Monday crude, homemade bombs and other suspected explosive devices have been addressed to Barack Obama, Hillary Clinton, CNN, liberal billionaire donor George Soros other figures loathed by Trump supporters.
Packages have been intercepted in New York, Maryland, Florida, Delaware and Los Angeles.
"It does remain possible that further packages could or have been mailed," assistant FBI director William Sweeney said, confirming that "some" packages had been sent through the US postal system. So far, no one has been hurt.
"We will identify and arrest the person or people responsible for these acts. Make no mistake," New York police chief James O'Neill told a news conference, refusing to divulge any leads investigators may have.
"It's an ongoing thing. We are discovering things by the hour," he added.
The US president on Thursday stepped up his attacks on the media, lashing out at the press for stirring up "anger". Democrats accuse the Republican leader of inciting violence that led to the attempted attacks.
In the early hours of Thursday, a retired police detective called the bomb squad to remove a suspected explosive device sent to double Oscar-winner De Niro at his Tribeca Productions company in New York.
The 75-year-old star used an expletive to condemn Trump at the televised Tony Awards last June, receiving a standing ovation.
Two additional packages were addressed to Obama's former vice president, Joe Biden, often rumored as a potential Democratic presidential candidate for 2020, in Delaware, the FBI confirmed.
'MEDIA MUST CLEAN UP'
With the United States bitterly divided and crucial midterm elections on November 6, Trump reacted first by calling for unity and condemning "acts of political violence," before reverting to attacks on the media.
"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," Trump tweeted Thursday.
"It has gotten so bad and hateful that it is beyond description. Mainstream Media must clean up its act, FAST!"
The White House was forced to defend the president over accusations that he was not taking the attempted bombings seriously enough.
"We condemn violence in all forms. This is a despicable act and certainly something that should never take place in America," Press Secretary Sarah Sanders told Fox News.
Critics accuse Trump's rhetoric-laden "Make America Great Again" presidency of emboldening right-wing extremists. He recently endorsed the body-slamming of a reporter, routinely denounces the press as "fake news" and has leveled toxic remarks in the past against the pipe bomb targets.
The packages were sent in manila envelopes with bubble wrap, marked with computer-printed address labels. Each listed Debbie Wasserman Schultz, former chairwoman of the Democratic National Committee, as the sender, including misspellings of her last name, the FBI said.
All of them have been sent for analysis to the same FBI laboratory in Virginia, officials confirmed Thursday.
US media report that at least some may have been sent from Florida, a battleground southern state home to Trump's lavish Mar-a-Lago estate that he dubbed the "Winter White House."
"I believe there is someone by definition a serial bomber, yes, and a terrorist," New York Mayor Bill de Blasio told CNN.
'FOREIGN OR DOMESTIC'
"Person or people, foreign or domestic -- we don't know. And we don't know if they're here or elsewhere in the country."
There has been no claim of responsibility and no known arrests.
Former CIA director John Brennan, who was sent the pipe bomb care of CNN, assailed Trump for attacking the media.
"Stop blaming others. Look in the mirror. Your inflammatory rhetoric, insults, lies, & encouragement of physical violence are disgraceful," he tweeted. "Your critics will not be intimidated into silence."
Top Democrat lawmakers Nancy Pelosi and Chuck Schumer have accused Trump of condoning "physical violence and dividing Americans."
CNN president Jeff Zucker, whose employees were forced to evacuate their New York bureau on Wednesday by the discovery of the pipe bomb, slammed the White House for its "complete lack of understanding" about "the seriousness of their continued attacks on the media."
White powder also sent to CNN presented no biological threat, Sweeney said.
Law enforcement agencies have called repeatedly on members of the public to come forward with any tips. They say calls to a hotline number are up 139 per cent as the investigation proceeds.