Suspected explosive devices have been sent to top Democrats including Barack Obama and Hillary Clinton, as well as CNN, US officials have said.
It comes two days after a pipe bomb was found at the home of liberal philanthropist and financier George Soros in the suburbs of New York City.
CNN’s New York office was evacuated on Wednesday morning after a suspected bomb was sent to their mailroom.
President Donald Trump said that these threats “have no place in America”.
Speaking at the White House, Mr Trump said that he has been briefed by the FBI and “a major federal investigation is now under way”.
“The safety of the American people is my highest and absolute priority,” he said.
“In these times we just have to unify. We need to come together and send one very clear unmistakable message that threats or acts of political violence of any kind have no place in America.”
Find out more about this development here.