Prince George's County prosecutors dropped charges against a Palmer Park man yesterday in the middle of his first-degree murder trial after one of the two main government witnesses took the Fifth Amendment and refused to testify and prosecutors could not find the second key witness.

The witness's unexpected refusal to testify came after the defense attorney in the case suggested that the witness, and not the defendant, was the actual gunman.

Circuit Court Judge Robert H. Mason granted Assistant State's Attorney Fran Longwell's request to dismiss the murder charge against Carl Anthony Cooper, 20, who had been charged with the March 20 shooting death of William King. King was fatally shot in a car in Palmer Park.

The dismissal after a jury had been sworn in to hear the case means that Cooper can never be tried in the slaying; to do so would constitute double-jeopardy, a violation of the Constitution's prohibition on being tried twice for the same crime.

The collapse of the case against Cooper was just the latest in a series of recent setbacks in murder cases for the office headed by Prince George's County State's Attorney Jack B. Johnson.

Johnson was out of town yesterday and unavailable for comment, a staff member said. Deputy State's Attorney Mark K. Spencer was out sick, and Johnson's spokeswoman, Paula Burr, did not respond to phone calls.

Two weeks ago, Prince George's prosecutors were criticized by some advocates for the local Latino community for their handling of a highly publicized case in which a Laurel teenager was accused of killing a Salvadoran immigrant.

In that trial, prosecutors did not allege that the crime occurred during a robbery--which could support a first-degree murder conviction--until their closing argument, after they had said in their opening statement that there was no motive for the crime.

In July, a mistrial was declared for a second time in a murder trial when, under questioning by Assistant State's Attorney Roland Patterson, a state witness testified that another man had been acquitted in the case. The judge ruled that that information could taint the jury's deliberations.

Douglas S. Wood, Cooper's attorney, said his client was "very relieved" by yesterday's dismissal.

In her opening statement, Longwell said Cooper approached a car King was in and shot him at point-blank range. King drove for a short distance, then crashed into a fence, mortally wounded, she said.

Prosecutors alleged that Cooper shot King out of revenge for an attack in late 1998 in which Cooper was shot and wounded.

In his opening statement, Wood agreed that King was shot out of revenge, but he argued that Cooper was not the culprit. Wood argued that Charles Harris, whose younger brother had been shot and wounded by someone from the Kentland area, shot King to avenge his sibling.

Charles Harris, 25, was called to the witness stand by Longwell shortly after opening statements. But when Longwell asked him to describe what happened the night of the slaying, he invoked his Fifth Amendment right, which allows people to refuse to testify if doing so may incriminate them.

The trial was scheduled to resume on Tuesday, but Mason granted Longwell a continuance so she could find another witness, Phyllis Magrum. Prosecutors said Magrum, who is not charged in the slaying, gave Cooper a ride after the shooting.

The murder charge was dismissed yesterday after Longwell could not locate Magrum.