Maryland state Sen. Clarence M. Mitchell III acknowledged yesterday that he had a loaded .38-caliber revolver in his briefcase when he tried to board a plane Saturday, but said he had grabbed the case without thinking from his car trunk.

Mitchell, speaking at a Baltimore press conference, said he was late for a flight at Baltimore-Washington International Airport that was to take him to Chicago. "While rushing from my car . . . I grabbed my attache case . . . not realizing my weapon was still in the case," he said. "It was never my intention to board the aircraft with the weapon."

When the pistol was detected by X-rays, state police confiscated it. Mitchell was permitted to take his flight. Mitchell said the pistol was legally registered with Baltimore City police, and that under the state gun-control law, as a businessman he does not need a permit to carry it. He said he owns a real estate company that was burglarized three times, which is why he keeps the pistol.

The decision to let Mitchell go was based in part on mistaken identity.

Anne Arundel County State's Attorney Warren Duckett said yesterday that when he told state troopers to confiscate the gun but let its owner go, he was under the mistaken impression that the man in question was U.S. Rep. Parren J. Mitchell (D-Md.). (Parren Mitchell is the state senator's uncle.)

Duckett said state police called his office to ask whether Mitchell, as a state senator, could claim immunity from permit requirements for the gun. But when the aide on duty relayed the message to Duckett, he said it was Rep. Parren Mitchell invoking immunity, Duckett said.

Duckett said he thought the issue of congressional immunity was at stake, while state senators have no such immunity. He also conceded: "In all honesty, if I'd known it was Clarence Mitchell I would have made the same decision."

He said, "I don't think Sen. Mitchell was taking the gun to hijack the plane or use it in any other way. As long as it was confiscated . . . we could allow the legal questions to be answered in court later."

Duckett said he would decide Wednesday whether to press charges.