"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.
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.