WASHINGTON (CNN) -- Sen. Jim Webb called the arrest of a top aide on weapons charges "extremely unfortunate" Tuesday after the aide was stopped as he brought the senator's loaded pistol into a Senate office building.

Phillip Thompson was arrested Monday afternoon on charges of carrying a pistol without a license, and having an unregistered firearm and unregistered ammunition, Capitol Police said. He faced arraignment Tuesday.

Webb, a freshman Democrat from Virginia, declined comment on details of Thompson's case, but called him "a longtime friend" and "a fine individual."

"He has worked for me since the beginning of the campaign last year," he said. "I have a tremendous amount of respect for him. I think this is one of those very unfortunate situations where, completely inadvertently, he took the weapon into the Senate yesterday."

Webb, who has a concealed handgun permit in Virginia and an A rating from the National Rifle Association, said he handed his gun off to aides before getting on a plane to New Orleans on Friday. He said he did not give Thompson the weapon directly, and was unsure how it ended up with him.

"We had three cars on Friday that were being moved about because of my trip, and that is probably a reason that this inadvertent situation developed," he said. "And that's really the extent to which I think I should be discussing."(Watch video: Webb comments on aide's arrest)

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-- CNN's Brianna Keilar, Eric Fiegel and Scott Anderson, and Emily McCulloch contributed to this report