Several package bombs sent to high-profile critics of President Donald Trump went through the processing and distribution centre in Opa-locka, law enforcement officials said Thursday.

The US Postal Service centre handles packages mailed from Broward, Miami-Dade and Monroe counties. The centre is on the southwest side of the Miami Opa-Locka Executive Airport, reports sun-sentinel.com.

Miami-Dade Police went to the mail-sorting facility on Thursday, said spokesman Argemis Colome.

‘Our bomb squad and K-9 Unit are currently providing assistance to our federal partners at the Opa-locka mail facility as part of the ongoing investigation into suspicious packages located in other jurisdictions,’ Colome said in a statement. ‘This assistance is as a precautionary measure. We cannot provide additional information at this time due to the active nature of this federal investigation.’

Trucker Todd Beckum of Palm Beach County makes regular runs between USPS distribution centres in West Palm Beach and Opa-locka, a route he has driven for four years.

‘It’s strange in all the years I’ve been doing this to have such a serious concern, to have this kind of concern in the air,’ he said while fuelling his rig at a station near the Opa-locka facility.

‘Postal inspectors, they’re in there doing what needs to be done. I have to rely on those around me I guess, because we’re all in this together.’

‘You hate the fact that something like this can go on in your own backyard,’ he said. ‘Not that you’d want it to happen to anyone else.’

Earlier Thursday, a law enforcement official told the Associated Press that investigators hunting for the source of the bombs suspect that some of the parcels were mailed from Florida.

The official said investigators have been examining a database of images of mail maintained by the US Postal Service. It has provided clues pointing investigators toward the state.

The official wasn’t authorised to discuss the investigation and spoke on condition of anonymity.

US Postal Inspection Service spokesman Philip Bartlett said postal employees and inspectors are searching mail facilities for packages matching the 10 sent over the past four days.

All of the packages contained the return address of South Florida congresswoman Debbie Wasserman Schultz.

One of the packages was delivered to her Florida office Wednesday when it was returned by the Postal Service. Her name was misspelled on the label. It was addressed to Eric Holder, former US attorney general under President Barack Obama.

Speaking at a campaign event in Plantation, Wasserman Schultz’s voice broke with emotion as she said her staff handled the threat with bravery.

Meanwhile, investigators searched coast to coast Thursday for the culprit and motives behind the bizarre mail-bomb plot, analyzing 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 — two addressed to former Vice President Joe Biden and one to actor Robert De Niro. Authorities warned there might 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 still uncertain whether the devices were poorly designed or never intended to cause physical harm.

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 news 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 Nov. 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, D.C., 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 Schultz, the former chairwoman of the Democratic National Committee.

The bombs seized Wednesday were about 6 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.

David Chipman, a retired federal ATF agent and now senior policy adviser for the Giffords Centre, said the details revealed telltale signs that could help guide investigators.

The tape on the pipe is ‘an investigator’s dream,’ he said, recalling a case in Texas that was solved because the fibres on the tape were traced to the bomber’s dog. He said bombers tend to plot methodically.

‘This is someone sitting down and spending time thinking about what they’re going to do to someone else. And some people like to relish that,’ he said.

The new packages discovered Thursday set off a new wave of alarm.

A retired New York police detective working in security in De Niro’s Manhattan office called police after seeing images of a package bomb sent to CNN and recalling a similar package addressed to the actor, officials said.

The packages addressed to Biden were intercepted at Delaware mail facilities in New Castle and Wilmington, according to a law enforcement official who, like others, wasn’t authorized to discuss the investigation and spoke on condition of anonymity.

Like earlier targets, both Biden and De Niro have been sharply critical of Trump. The actor dropped an expletive insult at Trump at this year’s Tony Awards and also apologized to Canadians for the ‘idiotic behavior of my president.’ Biden said last week that the president may not ‘know what he’s doing’ and coddles dictators.

Trump has called Biden ‘Crazy Joe’ and ‘mentally weak.’

On Thursday, during a campaign trip to suburban Buffalo, Biden said: ‘We’ve got to get off this hate machine. We’ve got to come together.’

The packages were ‘clearly an effort to terrorize people politically, to choose people for political purposes and attack them because of their beliefs,’ New York City Mayor Bill de Blasio said.

The first crude bomb to be discovered was delivered Monday to the suburban New York compound of George Soros, a liberal billionaire and major contributor to Democratic causes. Soros has called Trump’s presidency ‘dangerous.’

Similar packages addressed to Clinton and Obama were intercepted on their way to Clinton’s New York home where she lives with former President Bill Clinton and to Washington, where Obama lives with his wife, Michelle. The Secret Service said neither package reached its intended recipient.