The investigation into all 10 pipe bombs, which were sent to high-profile Democrats and critics of US President Donald Trump, is focusing on leads in Florida, a federal law enforcement source told Reuters.
Department spokeswoman Sarah Isgur Flores said authorities planned to announce more information at a press conference.
Federal investigators are searching for whomever sent a wave of suspicious packages addressed to political nemeses of President Trump.
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. It was found at a New York City postal facility and was similar to the other packages, the official added.
Among intended recipients earlier in the week were former Vice President Joe Biden, former Attorney General Eric Holder, actor Robert De Niro and former Central Intelligence Agency director John Brennan, whose security clearance Trump revoked after Brennan lambasted Trump's Russian Federation summit performance as "nothing short of treasonous". Officials said they do not think she had anything to do with the packages and consider her a likely target. The one discovered at a Manhattan postal facility was addressed to Clapper at CNN's address.
CBS News reported early on Thursday that an explosive device was also sent to former Vice President Joe Biden.
Attorney General Jeff Sessions said Friday the Justice Department was dedicating every available resource to the investigation "and I can tell you this: We will find the person or persons responsible".
Trump's comments come after tweets this morning alleging the bomb-like devices were shifting the national conversation away from early voting for the November 6 midterm elections.
Investigators have been searching coast to coast for the culprit and motives behind the weird mail bomb plot aimed at critics of the president, analyzing the innards of the crude devices to reveal whether they were meant to detonate or simply sow fear two weeks before Election Day.
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Investigators went to a Florida mail center on Thursday.
An additional clue: Misspellings from his online posts matched mistakes found on the packages, according to an 11-page criminal complaint that included the formal charges of threatening former presidents and transporting explosives across state lines. They bore the return address of Florida Rep. Debbie Wasserman Schultz, the former chairwoman of the Democratic National Committee who was accused by Clinton rivals of secretly helping the party's eventual presidential nominee.
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The common thread among them was obvious: critical words for Donald Trump and frequent, harsher criticism in return.
New York City officials urged New Yorkers to be vigilant in the coming days about unfamiliar packages, and to call police if they see anything suspicious.
At a press conference Thursday, officials in NY stressed they were still treating the packages as "live devices".
USA law enforcement is engaged in a frantic search for the person or persons who sent explosive devices in the mail to De Niro and other people.
The packages had numerous hallmarks of suspicious mail, including large block lettering and excessive postage aimed at making it harder to track, said Matthew Doherty, who formerly led the U.S. Secret Service's National Threat Assessment Center.
"These are not hoax devices", FBI Director Chris Wray said. Even with the sender still unknown, politicians from both parties used the threats to decry a toxic political climate and lay blame.
Trump tweeted on Thursday that much of the anger in society is caused by the "Mainstream Media".
At 2:48 a.m., he'd tweeted, "Funny how lowly rated CNN and others can criticize". Look in the mirror.
The bombs seized Wednesday were about 6 inches (15 centimeters) long and packed with powder and broken glass, according to a law enforcement official who viewed X-ray images. That package contained a device similar to one recovered from the offices of CNN on Wednesday.
The first crude bomb had been delivered on Monday to the NY compound of George Soros, a liberal billionaire and major contributor to Democratic causes.
The President on Wednesday condemned the attempted bombings and called for Americans to "unify" and strive for civility before stressing that the media "has a responsibility to set a civil tone".