A police bomb squad and canine units joined federal investigators on Thursday to examine a sprawling US mail distribution center at Opa-Locka, northwest of Miami, Miami-Dade County police said.

Homeland Security Secretary Kirstjen Nielsen confirmed that Florida appeared to be the starting point for at least some of the bomb shipments.

"Some of the packages went through the mail. They originated, some of them, from Florida," she said during an interview with Fox News Channel. "I am confident that this person or people will be brought to justice."

All the targets were figures frequently maligned by right-wing critics. They included Democratic Party donor George Soros, former President Barack Obama, former Secretary of State and presidential candidate Hillary Clinton, former Attorney General Eric Holder, former CIA director John Brennan and California Representative Maxine Waters. Two packages were sent to her.

The Federal Bureau of Investigation has said that at least five of the packages bore a return address for the Florida office of US Representative Debbie Wasserman Schultz, a former chair of the Democratic National Committee.

Holder's package ended up being rerouted and was delivered to the Wasserman Schultz return address. Brennan's package was sent in care of the Manhattan bureau of CNN, where he has appeared as an on-air analyst.

On Thursday, the investigation widened with the discovery of three additional packages. Two were intended for former Vice President Joe Biden in his home state of Delaware and one for the actor Robert De Niro in Manhattan.

Authorities believe the packages, which were intercepted before reaching their intended recipients, all went through the US Postal Service at some point, a source said. None detonated and no one has been hurt.

A federal law enforcement source told Reuters the devices were thought to have been fashioned from bomb-making designs widely available on the internet.

Investigators are nevertheless treating the devices as "live" explosives, not a hoax, said James O'Neill, police commissioner of New York City, where two of the parcels have surfaced.

Authorities have branded the parcel bombs an act of terrorism. They were sent less than two weeks before national elections that could alter the balance of power in Washington.

"It does remain possible that further packages have been or could be mailed," William Sweeney, assistant director of the FBI, told a news conference in New York.

Investigators have declined to say whether the devices were built to be functional. Bomb experts and security analysts say that based on their rudimentary construction it appeared they were more likely designed to sow fear rather than to kill.

The parcels each consisted of a manila envelope with a bubble-wrap interior containing "potentially destructive devices," the FBI said. Each was affixed with a computer-printed address label and six "forever" postage stamps, the agency said.

The episode sparked an outcry from Trump's critics, who charged that his inflammatory rhetoric against Democrats and the press was creating a climate for politically motivated violence.

After first calling for "unity" and civil discourse on Wednesday, Trump lashed out again Thursday at the "hateful" media. His supporters accused Democrats of unfairly suggesting the president was to blame for the bomb scare.

"Funny how lowly rated CNN, and others, can criticize me at will, even blaming me for the current spate of Bombs and ridiculously comparing this to September 11th and the Oklahoma City bombing, yet when I criticize them they go wild and scream, “it’s just not Presidential!” Trump said on Twitter at about 3:15 a.m. EST on Friday.

No one has claimed responsibility for the bombs, and the public was asked to report any tips.