The US Capitol building was briefly put on lockdown amid intense "police activity." People were told to take shelter in their offices, ensuring that the doors were locked. Reports suggest an armed person attempted to enter the building.

Law enforcement said they ordered the lockdown “because of police activity,” Reuters reports.

One Capitol Police officer told the news agency an “event” was under way and the authorities were looking for someone.

A Senate aide told Reuters that authorities were searching for a “possible female who could be armed in the subway of the Rayburn Building,” as he had heard on Capitol police officers' radios. Rayburn is connected to the US Capitol by an underground train often used by US representatives.

Police said no one would be allowed to leave or enter the building until the lockdown is lifted.

Capitol on lockdown. We're not being allowed in building. But officers seem calm. No large amounts of extra support pic.twitter.com/fpn7mn7hNc — Justin Gray (@grayjustin) July 8, 2016

A congressional staffer was seen with a firearm in the building, according to The Hill.

We're currently locked in the House chamber. Overheard a Capitol police officer say a woman entered Rayburn with a gun @CQnow — Katherine Scott (@KatherineBScott) July 8, 2016

Police gave an all-clear to reopen the Capitol building shortly after 9:30am (13:30 GMT), half an hour after the lockdown began. It was determined the person in question did not have a weapon, but carried a power drill and other tools in her backpack, Roll Call reported.

The atmosphere is currently tense due to Thursday's shooting in Dallas, in which at least five police officers were killed when some four attackers, holed up in a downtown garage, used sniper rifles against police officers during a protest against fatal police shootings. Seven more officers as well as two civilians were injured in the shootings. Police believe the attack was coordinated.

Capitol Police tell people in the Capitol to silence their phones and "close, lock and stay away from external doors and windows." — Eric Geller (@ericgeller) July 8, 2016

On @HouseFloor. Locked in as @CapitolPolice address security situation. God bless them and every officer across America who protects us. — Rep. Eric Swalwell (@RepSwalwell) July 8, 2016

It's not the first alert in the US Capitol building. In March, the building was on lockdown following an incident in which a 66-year-old man pulled a gun on an officer outside. In April 2015, the entire area around the Capitol was secured and searched after a man landed a gyrocopter on the west lawn of the building. That same month the institution was locked down after a man with a suitcase shot himself dead on the front steps.