Federal agents searched a US mail facility near Miami on Thursday night racing to find who sent 10 pipe bombs to prominent Democrats and critics of Donald Trump as leads pointed to Florida as the possible origin of the packages.

Investigators were 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 since the bomb scare began on Monday with a device addressed to billionaire Democratic Party donor George Soros.

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

Authorities have branded the parcel bombs, coming less than two weeks before national elections that could alter the balance of power on Capitol Hill, as an act of terrorism, though they have declined to say whether the devices were built to be functional.