After spending his 45th birthday in jail, Phillip Thompson, an aide to Sen. Jim Webb (D-Va.), appeared in D.C. Superior Court on Tuesday to plead not guilty to felony charges for carrying his boss’s loaded handgun into a Senate office building Monday.

Wearing a gray suit and a five o’clock shadow, Thompson — a retired Marine and longtime friend of Webb’s — was released on his own recognizance. He now faces a May court appearance and risks a maximum penalty of a $5,000 fine and up to five years in jail.

As they left the courthouse, both Thompson and his attorney, Richard Gardiner, a former general counsel for the National Rifle Association, gave no comment when questioned by reporters.

Meanwhile, Webb gave few additional details about the incident during a press conference at the Capitol on Tuesday. Webb did stress: “I have never carried a gun in the Capitol complex and I did not give the weapon to Phillip Thompson.”

Webb declined to discuss details of Monday’s incident, citing the pending court case and the fact that he was out of town when his executive assistant “completely inadvertently … took the weapon into the Senate yesterday.”

Webb added that Thompson’s possession of the handgun when he attempted to enter the Russell Senate Office Building on Monday morning likely was the result of a mix-up Friday when the Senator was preparing to fly to New Orleans.

“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.

Capitol Police officials, the Senate Sergeant-at-Arms and Webb himself all have stated that Thompson never had any intent to harm anyone when he brought the gun to Webb’s office building.

Thompson was taken into custody by police officers without incident at a door to the Russell Building after the pistol was discovered during a routine X-ray of a black briefcase Thompson was carrying.

According to Superior Court documents, Capitol Police working the door at 10:20 a.m. “observed what appeared to be an image of a gun and two magazines. … The defendant stated that he was in possession of a pistol and two magazines belonging to Senator Jim Webb.”

The report notes that Thompson said he had been “safekeeping” the weapon.

Thompson is charged with carrying a pistol without a license in violation of D.C. Code.

Although the D.C. prohibition against firearms was put into place in 1975, a provision in federal law gives Members of Congress and their staffs the right to bear arms on Capitol Hill.

According to Capitol Police Board regulations established in 1967, Members and their aides are allowed to transport licensed firearms on the Capitol grounds in the course of carrying out their official duties provided the weapons are “unloaded and securely wrapped.”

(Directives published in recent years also state that staff must be verified by Capitol Police.)

Although the regulations expressly prohibit weapons on the floor of either chamber, as well as in the adjacent lobbies, cloakrooms and galleries, individual Members are allowed to “maintain firearms within the confines of [their] office.”

Senate Majority Leader Harry Reid (D-Nev.) offered little comment on the incident Tuesday afternoon, saying simply, “I am aware of the issue, it’s a serious issue.”

Reid added that “everyone recognizes that [the aide] made a mistake.” But the Majority Leader wouldn’t elaborate further.

While noting that what has happened to Thompson is “enormously unfortunate,” Webb, who has a permit to carry a gun in Virginia, also maintained his strong support for gun rights on Tuesday.

“It’s important for me, personally and for a lot of people in the situation that I’m in, to be able to defend myself and my family,” he said.

He also noted providing for one’s own safety is something that members of the legislative branch have to think about more than officials in the executive branch.

“If you look at people in the executive branch, look at the number of people who are defending the president and other members of the executive branch. There is not that kind of protection available to people in the legislative branch. We are required to defend ourselves, and I choose to do so.”

One senior GOP leadership aide said Tuesday that the appropriate thing for Webb to do now would be to take some personal responsibility for Monday’s incident.

“The gun belonged to him, it was not the staffer’s weapon,” the aide said. “It’s hard to understand how he can try to distance himself from this situation because it all comes back to him. … The responsible thing to do and to end some of these questions would simply be to say, ‘Yes, it was my gun, it was loaded and things have not been handled well.’”

Meanwhile Andy Maybo, the chairman of the Capitol Police Labor Committee, praised his fellow officers for their efforts to protect the Capitol on Monday.

“While the circumstances surrounding the discovery of a firearm by members of the United States Capitol Police remain under investigation, the actions of the Capitol Police officers who made the discovery and subsequent arrest deserve commendation,” Maybo said.

“The officers were unflinching and reacted professionally as they secured the weapon. No one was harmed nor was there any undue disturbance during the discovery and arrest,” he added.

Erin P. Billings contributed to this report.