The two bombs sent to former Vice President Joe Biden and a third to actor Robert De Niro on Thursday were similar to the devices intended for several other high-profile Democrats and critics of U.S. President Donald Trump, authorities said.

None of the 10 devices exploded but authorities stepped up a manhunt for the serial bomber amid 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 and be vigilant.