WASHINGTON (AP) — A man sitting in a pickup truck outside the Library of Congress told police on Thursday that he had a bomb, prompting a massive law enforcement response to determine whether it was an operable explosive device, people briefed on the matter said.

The truck had no license plates, and when law enforcement noticed it in the morning, authorities reported a possible bomb threat over police radios. Investigators were trying to determine whether the man was holding a detonator. They were communicating with him as he wrote notes and showed them to police from inside the truck, according to three people were not authorized to publicly discuss the matter and spoke on condition of anonymity.

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.

Capitol Police said officers were “responding to a suspicious vehicle near the Library of Congress,” and that it was an “active bomb threat investigation.”

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