WASHINGTON — A man sitting in a black pickup truck outside the Library of Congress told police Thursday that he had a bomb, prompting a massive law enforcement response to determine whether it was an operable device, law enforcement authorities said.

Police evacuated multiple buildings around the Capitol after officers observed the man holding what appeared to be a detonator, U.S. Capitol Police Chief J. Thomas Manger said. The man's name was not immediately disclosed.

Police negotiators were communicating with him as he wrote notes and showed them to authorities from inside the truck, according to three people who were not authorized to publicly discuss the matter and spoke on condition of anonymity.

"My negotiators are hard at work trying to have a peaceful resolution to this incident," Manger said. "We're trying to get as much information as we can to find a way to peacefully resolve this."

The episode began about 9:15 a.m. when the truck drove up the sidewalk outside the library, Manger said. The driver told the responding officer that he had a bomb, and was holding what the officer believed to be a detonator, The truck had no license plates.

Police sent snipers to the area near the Capitol and Supreme Court and evacuated multiple buildings on the sprawling Capitol complex. Congress is in recess this week, but staffers were seen calmly walking out of the area at the direction of authorities. 

The nation's capital has been tense since the Jan. 6 insurrection at the Capitol by supporters of then-President Donald Trump.

Fencing that had been installed around the Capitol grounds had been up for months but was taken down this summer. A day before thousands of pro-Trump rioters stormed the Capitol, pipe bombs were left at the headquarters of the Democratic National Committee and the Republican National Committee in Washington. No one has been arrested yet for placing the bombs.

The RNC, not far away from where the truck was parked Thursday, was also evacuated over the threat. 

The area was blocked off by police cars and barricades, and multiple fire trucks and ambulances were staged nearby. Also responding were the District of Columbia's Metropolitan Police, FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives. 

The White House said it was monitoring the situation and was being briefed by law enforcement.