U.S. Capitol On Lockdown

6:45 AM PT -- Police have ended the lockdown. Cops say an employee had passed through security with an item in a backup that appeared to be a weapon.

Once they determined it was not ... everything went back to normal.

The U.S. Capitol is on lockdown while police search for an individual -- possibly armed -- and people already in the building have been told to take shelter.

According to law enforcement there was a report of a person with a gun in a subway near Capitol. It's unclear if the person has actually entered the Capitol.

Right now authorities are saying the lockdown is being done out of "an abundance of caution."

Story developing ...