After having evacuated 10,000 staff from Congressional office buildings surrounding Capitol Hill, the police began hunting down the suspcious characters who had sparked the massive security alert.
A SWAT team - police specialists - stormed into the building at one point.
Police earlier released reports saying a white male of about 20 years of age had got inside the premises with a gun. He was said to be accompanied by a white female.
According to the earlier version of the police, the man put his backpack through a metal detector upon entering the Cannon building.
Security officers saw something that looked like a revolver and moved to question him, but before the guards could stop him, the man grabbed the bag and ran into the building.
It led to a security alert that saw 10,000 staff being evacuated from Congressional buildings surrounding Capitol Hill.
"We do not know where he is, we are doing a systematic floor-by-floor search and evacuation at the same time," Congressman Bob Ney told reporters.
Business in the Capitol's chambers were continuing uninterrupted, but underground tunnels linking the Cannon office building to the Capitol complex have been sealed.
Within an hour Capitol police chief Terrance Gainer called off the alert when it emerged two female congressional staff had caused the scare.
"It was just an unusual set of Halloween circumstances. I don't think they had any ill intent. They're very sorry this all happened," said Gainer.