The investigation into 10 pipe bombs sent to high-profile Democrats and critics of US President Donald Trump is focusing on leads in Florida, a federal law enforcement source said on Thursday, while another source said investigators believe the devices' design came from the internet.

All the suspicious packages were believed to have gone through the US postal system at some point, the source said. Many of the packages had the return address of the Florida office of US Representative Debbie Wasserman Schultz, who formerly chaired the Democratic National Committee, according to the FBI.

The instructions for such explosive devices are widely available on websites and in propaganda distributed by militant groups such as Islamic State and al-Qaeda, said the second law enforcement official and a former federal government bomb expert.

Former vice-president Joe Biden and actor Robert De Niro on Thursday became the latest targets of the bombs, none of which have exploded.

The bombs have heightened what already was a contentious campaign season ahead of the Nov 6 elections in which Trump's Republicans will try to maintain majorities in the Senate and House of Representatives.

No one has claimed responsibility and the FBI urged the public to report any tips. The packages consisted of a manila envelope with a bubble-wrap interior containing "potentially destructive devices," the FBI said. Each bore a computer-printed address label and six "Forever" postage stamps, the agency said.

Some of the packages sent to New York locations had envelopes of white powder in them, but FBI Assistant Director William Sweeney said at a briefing in New York that initial analysis showed it was not a biological threat.

Authorities described the devices as crude while security experts said their goal may have been to create fear rather than to kill.

Leading Democrats said the bombs were a dangerous outgrowth of the antagonistic political atmosphere surrounding the upcoming mid-term elections.

Trump and other Republicans have likened Democrats to an "angry mob," citing protests at the confirmation hearing of US Supreme Court Justice Brett Kavanaugh.

The device intended for De Niro, who hurled an obscenity at Trump at the Tony Awards in June, was sent to one of his properties in New York City.

"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 wrote. "It has gotten so bad and hateful that it is beyond description. Mainstream Media must clean up its act, FAST!"

Several of the people who were sent bombs — including former President Barack Obama and Trump's rival for the presidency, Hillary Clinton — are frequent targets of right-wing critics.

Also sent packages were Eric Holder, who served as attorney general under Obama; former CIA Director John Brennan, Democratic Party donor George Soros and California Representative Maxine Waters, an outspoken Trump critic, who was sent two packages.

Several politicians, including US Senate Majority Leader Mitch McConnell and New York City Mayor Bill de Blasio, described the package bombs as an act of terrorism.

"Someone is trying to intimidate. Someone is trying to quash voices in this country using violence," De Blasio said. "I am confident that we will find the perpetrator or perpetrators."

REUTERS