WASHINGTON (CNN) -- An aide to Sen. Jim Webb, D-Virginia, was arrested Monday morning with a gun at the Russell Senate Office Building, according to Capitol Hill Police.

Webb's office identified the employee as Phillip Thompson. A statement from Capitol Police Sgt. Kimberly Schneider indicated an X-ray machine spotted "a loaded pistol with two fully loaded magazines" when Thompson entered the building shortly before 11 a.m.

"It was determined that he did not have a license to carry a pistol in the District of Columbia," Schneider said. "He was in possession of an unregistered firearm and unregistered ammunition. He was subsequently placed under arrest and charged with carrying a pistol without a license, unregistered firearm and unregistered ammunition."

Details from Webb's office were sketchy. In a written statement, Webb communications director Jessica Smith said, "To our knowledge, this incident was an oversight by the senator's aide. Phillip Thompson is a former Marine and is a long-term trusted employee of the senator. We are still awaiting the facts."

A congressional official who has been briefed on the incident by law enforcement told CNN that the handgun belonged to the senator. Thompson was driving Webb to a Washington-area airport, located in Virginia, earlier Monday morning, the official said, when Webb remembered that he had the gun on him. It is legal to carry a concealed handgun in Virginia.

He gave the weapon to Thompson, the official said, and asked him to take care of it. Thompson subsequently put the gun in a briefcase and returned to Capitol Hill where "he spaced, he literally forgot" that he had the gun, the official said.