Youth, Mother Tell Court of Seeing Officer Slain : Violence: Teen-ager demonstrates how Compton policeman was shot in head. San Pedro man is standing trial for 1993 killing, in which second officer also died.

Holding a black 9-millimeter pistol in both hands, the 14-year-old boy took careful aim at the prosecutor’s head. Deputy Dist. Atty. Mark Arnold lay still, face down on the courtroom floor. The boy stood over him, straddling his back.

“It was like that,� the teen-ager said quietly.

In the front row, the parents of Compton Police Officer James MacDonald winced visibly as they watched the dramatic re-creation of their son’s 1993 murder.

The chilling scene was played out Monday by one of two eyewitnesses who told Superior Court jurors how MacDonald and his partner, Kevin Burrell, were gunned down while making a routine, late-night traffic stop.

On trial for one of the most shocking police killings in Los Angeles County in recent memory is Regis Deon Thomas, 23, of San Pedro, who has pleaded not guilty to both murders. If found guilty, he faces the death penalty.

Monday’s witnesses--the teen-ager and his mother, their anonymity protected by a court order prohibiting the news media from identifying them--described the murders as a series of snapshots.

From their moving car on Rosecrans Avenue shortly after 11 p.m. on Feb. 22, 1993, they first saw the officers struggling to put the suspect--a man they have not been able to positively identify--in handcuffs. Then came the shots, fired just a few feet from the family car. Finally, through the rear window, the boy testified he saw an unidentified man straddling MacDonald with a gun pointed at his head. The gun spit sparks as it fired, the boy said.

“I said, ‘They’re shooting him in his head,’ � the soft-spoken boy said. “We were all scared.� The two girls in the back seat “were screaming.�

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The mother and son, who police say have received threats warning them not to testify, are two of four important witnesses in the case against Thomas. But they may be the only witnesses to take the stand, prosecutors said Monday.

A young woman who was riding with the mother and son has apparently fled to avoid testifying, police say. Another witness, Calvin Cooksey--who has said that he heard Thomas boast of the killings and helped him get rid of the gun--has refused to return from his new home in Arizona for the trial so far, despite a court order and arrest warrant. He says he believes his life is in danger.

Late Monday, however, Cooksey’s attorney in Arizona said Cooksey may turn himself in to the court today. Cooksey has sued Compton and Los Angeles County for negligence, saying his mother was shot to death in an act of retaliation against him for his testimony and blaming police for not relocating her as promised. Authorities have denied the allegations.

Thomas, who is also on trial for allegedly killing a man over a perceived slight in 1992, is believed to be well-connected with a local gang that may seek revenge, said Compton Detective Marvin Branscom.

Superior Court Judge Edward A. Ferns has given prosecutors permission to show a tape of Cooksey’s testimony from the preliminary hearing if Cooksey does not show up. Of the four key witnesses, only Cooksey firmly ties Thomas to the crime.

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Ferns appeared to be less willing to allow previously recorded testimony by the young woman. Even if the tape is shown to jurors, it may be shown toward the trial’s end, Ferns said.

“My advice would be to send out the troops--whether that is the Sheriff’s Department or the Compton Police Department--to find her,� Ferns said.

Fear has been a dominant force among witnesses in this case. Despite intense media attention and pleas from law enforcement and political leaders, the mother, son and young woman who witnessed the shooting took five days to come forward and tell police what they had seen, detectives said.

The three were driving to pick up the mother’s oldest son from work. When asked why the three, plus a toddler, went to pick up the older son, the 14-year-old simply said, “My mother never leaves us home by ourselves.�

All of the eyewitnesses described a suspect with a haircut, build and complexion like Thomas’, but none could say, without reservation, he was the one.

The trial is expected to last at least six weeks.