Gunfire from a passing pickup truck killed a woman motorist in Alexandria and seriously wounded a second driver on the Woodrow Wilson Bridge early yesterday. Police arrested a suspect in the woman's slaying last night and said they believe the two shootings and a third near the bridge, in which a motorist escaped injury, were connected.

The suspect, arrested by Maryland State Police, was identified as Phillip Joseph Steeves, a 21-year-old Marine corporal stationed at the Marine Barracks in Southeast Washington. Steeves surrendered at a service station in Landover Hills at 7:35 p.m. under an arrangement worked out through his commanding officer, police said.

The three early morning shooting incidents occurred within one hour and three miles of each other, and a witness saw a Marine Corp decal on the pickup truck, authorities said. It was a police lookout for that truck -- described as brown, having a camper top and Kansas license plates -- that led to the arrest, according to a law enforcement source.

Military police, alerted by that lookout, spotted such a vehicle entering the barracks at Eighth and I streets SE about 4 a.m., according to the source, who said the truck's owner later told investigators he had lent it to Steeves.

After Steeves was taken into custody last night, Alexandria police, who had obtained a warrant charging him with murder, said they still did not know the motive for the slaying of Mary Ann Montecalvo, 24, of Arlington, who was shot in the chest as she and her roommate drove north in the 1200 block of N. Henry Street shortly after midnight.

The roommate, Claire Hunt, 20, said she was resting in the front passenger seat when she heard an explosion and a scream and saw Montecalvo fall into her lap.

A sniper, riding alongside the women's car, fired a single shot through the closed window on the passenger side with a .44-caliber magnum handgun, according to police. The bullet narrowly missed Hunt, who managed to stop the car.

Police said they learned of the fatal shooting of Montecalvo first, although they now believe it was the last of the incidents. In addition to that shooting, the wounding on the Wilson Bridge and a third one on Rte. I-95 on the Maryland approach to the bridge, police also were investigating another shooting incident in the District of Columbia and a midmorning incident in Montgomery County in which an Oxon Hill man was grazed by a bullet as he walked along a sidewalk in Silver Spring.

It was not known last night whether the last two incidents might be related to the other three. Police said investigation of all the shootings was continuing.

Steeves' surrender came by arrangement at a BP service station at 6815 Annapolis Rd. in Landover Hills. He was accompanied by his commanding officier, Marine Maj. Gilford G. Robinson, and was taken into custody on a fugitive charge by Maryland State Police as representatives of Alexandria police, Virginia State Police and the Naval Investigative Service looked on, according to a Maryland State Police spokesman.

The surrender was arranged after Maj. Robinson, whom Steeves had contacted through a friend, called Virginia authorities about 4:30 p.m. to say that Steeves wanted to turn himself in, police said.

Steeves, whose parents live in La Grande, Ore., lives in the Bellevue military housing area at Bolling Air Force Base and is assigned to the Food Services and Recreation Division of Headquarters Company at the Washington Marine Barracks.

At a hearing late last night, Prince George's County Circuit Judge Vincent Femia ordered Steeves held under $100,000 bond. A hearing on his possible extradition to Alexandria to face the murder charge there was scheduled for this morning.

Police were first summoned to the scene of the Montecalvo shooting early yesterday after a truck driver who had been following Montecalvo's blue Fiat saw it suddenly swerve to the right and stop. Believing the car had been in an accident, the truck driver radioed his dispatcher, who called Alexandria police.

Less than 10 minutes later, Virginia State Police were summoned to the Woodrow Wilson Bridge over the Potomac, about a mile from the scene of the Montecalvo shooting. There they found Marc-Gerald L. Muntz, 28, of 8081 Janna Lee Dr. in Alexandria, who had been shot in the chest while driving westbound toward Virginia.

Police said they believe that incident occurred about 30 minutes before it was reported, a delay they attribute to the relative difficulty of finding a telephone near the bridge, which links Maryland and Virginia.

Muntz was admitted to the intensive care unit of Mount Vernon Hospital, where he was listed in serious condition.

Shortly before Muntz was shot, the first shooting incident occurred less than a mile away on I-295 a short distance from the bridge.

Donald Gee was driving home to Stafford County when his car was reportedly struck by a bullet. Police said Gee sped off toward Virginia. When he reached his home, Gee notified state police and provided them with a description of the pickup truck.

Hunt, the passenger in Montecalvo's car, said yesterday she neither saw nor heard anything unusual before the shooting. "I thought it was some wires in the car that had exploded," recall Hunt, interviewed yesterday in the one-bedroom garden apartment the women shared near Fort Myer.

She said the two were returning home from a friend's apartment in Alexandria at the time of the incident.Hunt said the passenger seat in which she was sitting was broken and she was sitting in a reclining position out of the path of the bullet, a fragment of which cut her lip.

After hearing her roommate scream and slump against her, Hunt said she "kicked Mary's feet out of the way and slammed on the brake and took the steering wheel and pulled it in the opposite direction. When I went to lift Mary off me, there was blood all over me."

Hunt said she got out of the car, ran around to the driver's side and opened the door. "I went for her wrist and there was a very rapid pulse," said Hunt, who them stood behind the car waiting for police to arrive. "I kept thinking, 'Oh God, please don't let this happen.'"

Police said Montecalvo, who worked as an assistant manager of a clothing store at Prince George's Plaza, was prounced dead at Alexandria Hospital. Hunt was treated for minor injuries and released.

Friends and relatives of Montecalvo, who grew up in a small Pennsylvania town near the New York State border, expressed shock and anger at her death.

"Mary Ann never had an enemy in the world, she loved to go out and party," recalled her 22-year-old brother Vinnie, reached by telephone at the family's home in Bradford, Pa. "I was just getting up a 5:30 to go to work when the phone rang and my dad answered it."

"I heard him say, 'oh, no,' and I asked him what happened and he said some guy had pulled up in a car and shot Mary Ann. Then he looked at me and said 'I don't know who to call, what to do.' It was the first time I'd ever seen him cry."

"Our family is pretty close-knit and my Mom and Dad didn't like her being in the city, so far away from home. They thought Washington was a pretty dangerous place," Montecalvo recalled. "This makes me mad but there's nothing I can do about it. It just seems so senseless."