Marine Sgt. Zachary Cooper had both a wife and a girlfriend he had promised to marry. But a desire to pursue a third romance led him to kill both women and his 5-year-old daughter, Arlington County prosecutors said yesterday.

During the opening day of Cooper's capital murder trial in Arlington County Circuit Court, Deputy Commonwealth's Attorney Theo Stamos said the 24-year-old lured his wife, Maya, 21, and his girlfriend Marie Gault, 20, to the Cherry Blossom Travelodge on Jan. 20, 2002. In a third-floor room, Stamos said, Cooper fatally shot his wife and his 5-year-old daughter, Dessire. He then stabbed and shot Gault, Stamos said.

In the days leading up to the shooting, "Cooper's life was getting very complicated," Stamos said. "Slowly, a plan emerged."

But defense attorneys yesterday painted a different picture of what happened in Room 327 that cold Sunday evening.

Cooper killed Gault, his girlfriend, attorney Janell Wolfe acknowledged to jurors -- but only after she had shot his wife and child.

"Marie said something to the effect of, 'Now we can be together,' " Wolfe said. "He did go into a frenzy, and he did stab her with the knife. He also picked up the gun and shot her once in the head."

Jurors, who must decide which version to believe, heard the first evidence yesterday in a trial that lawyers said could stretch into next week.

Cooper, who is being held without bond, faces two capital murder charges in the death of Dessire. Virginia law allows a capital murder charge when the victim is younger than 14 or part of a multiple homicide. Cooper faces two additional capital murder charges in the slayings of his wife and girlfriend.

Arlington County Commonwealth's Attorney Richard E. Trodden said yesterday that if he wins a conviction, he will "present the option" of a death sentence to the jury.

Shaunta McCoy, who had checked into Room 326, told jurors she had just turned on the "Sopranos" television show when she heard a man and woman arguing in the next room. McCoy said that the woman began screaming, "No!" and "Help!" and that she called the front desk and asked the manager to call 911.

"I started hearing the little girl scream," McCoy said. "That's when I called the second time."

Arlington Police Cpl. William Couvillon, the first officer to arrive, testified that he heard muffled voices and then a pop that sounded like a gunshot as he approached in the hall.

Couvillon said he drew his gun and went into the room. He saw Cooper, his hands covered in blood, lean against the wall and slide to the floor. "He was repeatedly saying, 'Please help me,' " Couvillon said.

Couvillon said he saw two women lying face down on the floor in pools of blood. A toddler -- Gault's daughter -- clung to the back of one of the women, he said. Dessire, who had been shot in the chest as she watched the Disney Channel, was sitting, mortally wounded, on a chair across the room, Stamos said. The toddler was not wounded and is living with an aunt.

In her opening statement, Stamos told jurors that a series of stormy relationships led Zachary Cooper to that day in the motel room.

Cooper was living with his wife, Maya, and their young son on the Army post at Fort Knox, Ky., Stamos said. Dessire and Zachary Jr., Zachary Cooper's children from a high school romance, also lived with them. The weekend of the killings, Maya Cooper was in Northern Virginia visiting her grandmother.

Zachary Cooper had maintained a longtime relationship with Gault, who lived in Northern Virginia, Stamos said. During the preceding Thanksgiving holiday, they had shopped together at a mall and he had bought her a diamond ring. "He convinced Marie that the two of them were going to be together," Stamos said.

Then, Stamos said, Cooper struck up a relationship with Cartrice Mullins, a neighbor in Kentucky, and the two "started planning a life together."

The weekend of the killings, Cooper checked into Room 327 and Gault checked into another room. Cooper picked up his wife at her grandmother's house in Arlington and took her to the motel with no suitcase or even a purse, the grandmother, Barbara McDaniel, 62, testified yesterday.

When Cooper walked into the motel, Stamos said, he carried one bag.

"It's not clothes, it's not a toiletry bag. It's a bag with gun and ammunition," Stamos said. Cooper also had a knife in a sheath strapped to his right leg.

Wolfe said Cooper, who had received awards for his service in the Marine Corps, brought his gun inside because he didn't want to leave it in his van. She said he was in the shower when he heard gunshots and stepped out to find his wife and daughter shot.

"Zachary Cooper is responsible for Marie Gault's death . . . I'm telling you right up front," Wolfe said. "But I'm going to ask you to acquit him on all other charges."