US investigators search for person who sent mail bombs, motive

US investigators have been searching coast-to-coast for the culprit and motives behind the bizarre mail-bomb plot aimed at critics of the president, analysing the innards of the crude devices to reveal whether they were intended to detonate or simply sow fear two weeks before Election Day.

Three more devices were linked to the plot Thursday — two addressed to former Vice President Joe Biden and one to actor Robert De Niro — bringing the total to 10 in an outbreak of politically loaded menace with little if any precedent. Authorities warned there might well be more.

Law enforcement officials told The Associated Press that the devices, containing timers and batteries, were not rigged like booby-trapped package bombs that would explode upon opening.

But they were uncertain whether the devices were poorly designed or never intended to cause physical harm. A search of a postal database suggested at least some may have been mailed from Florida, one official said. Investigators are homing in on a postal facility in Opa-locka, Florida, where they believe some of the packages originated, another official said.

The officials spoke on condition of anonymity because they were not authorised to discuss the ongoing investigation by name. Homeland Security Secretary Kirstjen Nielsen, in an interview Thursday night with Fox News Channel, acknowledged that some of packages originated in Florida.

New details about the devices came as the four-day mail-bomb scare spread nationwide, drawing investigators from dozens of federal, state and local agencies in the effort to identify one or more perpetrators.

The targets have included former President Barack Obama, Hillary Clinton, CNN and Rep Maxine Waters of California. The common thread among them was obvious: critical words for Donald Trump and frequent, harsher criticism in return.

At a press conference Thursday, officials in New York would not discuss possible motives, or details on how the packages found their way into the US postal system. Nor would they say why none of the packages had detonated, but they stressed they were still treating them as "live devices."

"As far as a hoax device, we're not treating it that way," said Police Commissioner James O'Neill. Details suggested a pattern — that the items were packaged in manila envelopes, addressed to prominent Trump critics and carried US postage stamps.

The devices were being examined by technicians at the FBI's forensic lab in Quantico, Virginia. The packages stoked nationwide tensions and fears as voters prepared to vote November 6 to determine partisan control of Congress — a campaign both parties have described in near-apocalyptic terms.

Even with the sender still unknown, politicians from both parties used the threats to decry a toxic political climate and lay blame. "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 said on Twitter.

"It has gotten so bad and hateful that it is beyond description. Mainstream Media must clean up its act, FAST!" Former CIA Director John Brennan, the target of a package sent to CNN, fired back.

"Stop blaming others. Look in the mirror," Brennan tweeted. "Your inflammatory rhetoric, insults, lies, & encouragement of physical violence are disgraceful. Clean up your act....try to act Presidential." The list of bombing targets spread from New York, Delaware and Washington DC, to Florida and California.

The explosive devices were packed in envelopes with bubble-wrap interiors bearing six American flag stamps and the return address of Florida Rep Debbie Wasserman Schultz, the former chairwoman of the Democratic National Committee.

The bombs seized Wednesday were about six inches long and packed with powder and broken glass, according to a law enforcement official who viewed X-ray images. The official said the devices were made from PVC pipe and covered with black tape.

At the New York briefing, authorities confirmed that at least some of the packages were distributed through the US mail, and cautioned that there could be additional devices in the postal system.

They said investigators searching for more suspicious parcels had not found any during the previous eight hours.