Violent threats aimed at lawmakers ahead of former President Donald Trump‘s impeachment trial have prompted fears among federal law enforcement officials that armed protesters could return to wreak havoc in the U.S. Capitol.

An unnamed U.S. official told the Associated Press on Sunday that law enforcement were probing threats to kill or attack congressional lawmakers near the Capitol building ahead of Trump’s Senate impeachment trial. The source spoke on condition of anonymity as they were not allowed to publicly discuss an ongoing investigation.

The threats, which were mostly shared online and in chat threads, included plans to attack congressional lawmakers as they’re traveling to and from the Capitol building to attend the impeachment proceedings, according to the official.

Capitol police and other federal authorities have insisted that thousands of National Guard troops remain in Washington D.C. throughout Trump’s impeachment trial to address potential violence following the threats, the official said.

Newsweek reached out to the Department of Defense for comment.

More than 25,000 members of the National Guard were deployed to D.C. to secure President Joe Biden‘s inauguration in the aftermath of the deadly Capitol riot by a pro-Trump mob on January 6. Around 7,000 troops are expected to remain in the district until February, and 5,000 will stay until at least mid-March.

Following the Capitol insurrection, officials feared that another attack could occur before or during Biden’s inauguration, but the ceremony passed with few disruptions after the increased security measures were put in place.

This is a developing story.