Updated

U.S. Capitol Police arrested a top aide to Sen. Jim Webb on Monday after he tried to enter a Senate office building carrying a loaded pistol and two fully loaded magazines that belonged to the senator.

Phillip Thompson sent a bag through the X-ray machine at Russell Senate Office Building, where Webb's office is located. It detected the weapon and Capitol Police say they determined that Thompson didn't have a license to carry the gun in Washington, D.C. Thompson was arrested and charged with carrying a pistol without a license and possession of an unregistered firearm and unregistered ammunition.

A senior Democratic aide said Webb gave the bag that contained the gun to Thompson when the aide drove the senator to the airport. Thompson said he forgot it was in the bag when he took it into the office building.

"To our knowledge, this incident was an oversight by the senator’s aide. Phillip Thompson is a former Marine, a long-term friend and trusted employee of the senator. We are still awaiting facts," Webb Communications Director Jessica Smith said in a statement.

Thompson, a former military reporter based in Virginia, joined the senator's staff after Webb was elected to office in November. He travels frequently with the senator.

Handguns are illegal in Washington, D.C., but nearby Virginia allows residents to carry concealed handguns. Capitol Police rules allow members and their employees to bring a weapon onto Capitol grounds if it is unloaded and securely wrapped. In this case, it was neither.

Webb told The Richmond Times Dispatch in December that he considers himself a "strong Second Amendment person" but said he does not carry a gun to work.

"I don't carry personally regularly. I shoot regularly. I like to have the option to carry, which is why I keep the permit," he said. "I value the opportunity to be able to protect myself, depending on situations. But no, I don't walk around with a weapon on me all of the time," he told the newspaper.

Click here to read The Richmond Times Dispatch interview.

FOX News' Jim Angle and Trish Turner contributed to this report.