A Maryland man shot to death Thursday by an Andrews Air Force Base security guard may have been taking an unauthorized but familiar shortcut through the base when the guard stopped him and a dispute began, according to the dead man's mother and a friend who witnessed part of the incident.

The Air Force and FBI said yesterday that Mark Francis Nicholas, 34, a truck driver from Upper Marlboro, drove through the base's West Gate about 12:30 p.m. without showing a pass, and behaved "erratically" when the guard stopped him moments later. They said he was killed by a single bullet from the guard's 9mm pistol during "a confrontation," but would not elaborate.

Despite the tension in the Persian Gulf, Air Force officials said the base was not on a heightened alert status.

Officials said they expect an autopsy report to show whether Nicholas was under the influence of drugs or alcohol at the time of the incident. Nicholas's mother, in an interview, said her son had been struggling on and off with a drug problem since about age 12. Two friends, meanwhile, said his behavior had been "weird" in recent days.

His mother, Morina, said she and her husband, Frank, a retired Navy petty officer, have military-issue stickers on their cars, allowing them access to the 5,000-acre base. She said they often follow a shortcut through Andrews to their home on Midland Turn in Upper Marlboro.

She said her son had taken the shortcut many times in his parents' cars, but had never been issued a sticker for his own 1969 Ford pickup.

Nicholas, who lived with his parents, was driving the pickup Thursday, and was following the shortcut when the security guard stopped him in the parking lot of the base's Malcolm Grow Medical Center annex.

"He knew his way all around that base," his mother said, adding that her son, as a child, had been treated at the medical center's emergency room several times. "That's where we'd bring him, since my husband's retired military. All his records should be in there from when he was a child."

Nicholas was pronounced dead in the emergency room after the shooting. His mother said doctors told her the bullet pierced his heart.

Officials declined to say whether Nicholas was armed. His parents said he did not own a gun. A friend who was riding with Nicholas in the pickup, and who got out before it passed the West Gate, said Nicholas was carrying a screwdriver in his back pocket when he entered the base.

The friend, Earl Calloway, 18, of Clinton, said that while being interviewed by Air Force officials after the shooting, he saw one of them carrying the screwdriver in an evidence bag.

Calloway said he and Nicholas were using the pickup Thursday to carry a waterbed from Calloway's home to the home of an acquaintance in Suitland. They were headed south on Route 5 near the base when traffic slowed. He said Nicholas turned left on Auth Road and made his way to Andrews's West Gate.

"I kept asking him where he was going," Calloway said. "He was acting really weird, just mumbling to himself."

At the open gate, Nicholas stopped and saluted a security guard who emerged from a guard shack and walked toward the pickup, Calloway said. "I wasn't sure I was supposed to be there, so I got out of the truck."

Nicholas, without waiting, drove onto the base. "The guard wasn't too excited," Calloway said. "He just said on the radio, 'This is Police 14. We have a maroon pickup that didn't show ID.' "

The pickup turned right onto Perimeter Road, the route of the family's shortcut. Had Nicholas stayed on Perimeter, he eventually would have reached the base's Virginia Avenue Gate, which leads back to Route 5, about three miles south of where Nicholas first turned off the road.

But a security guard stopped him before he had traveled a half mile.

Listening to Air Force police radio chatter back at the guard shack, Calloway said, he heard the security guard say, "I'm going to need a back up for this guy."

Seconds later, Calloway said, he heard the shot.

Nicholas, who injured his neck last month and was on medical leave from his job as a moving van driver, underwent treatment for drug addiction several times in his life, his mother said. She said her son was arrested more than once as a juvenile, and served at least two jail terms as an adult.

Prince George's Circuit Court records show convictions for housebreaking in 1981, robbery in 1986 and assault in 1987.

"He was always hanging out with the wrong crowd," his mother said.

Yet Morina Nicholas, sitting in her kitchen yesterday, said her son had managed to hold a steady job with a trucking company in the last few years. She rummaged in a drawer and came out with an envelope of snapshots.

"There he is with that moving van of his," she said, pointing to one of the photos. "Drove a big 18-wheeler. Went to school for it. Cost us $2,900."