A Prince Georgehs County policeman testified yesterday that Terrence G. Johnson looked "cool and calm, not scared or worried" when he fired the shot that killed Officer James Brian Swart in the Hyattsville Police Station last June.

Officer Steven J. Roberts, testifying at Johnson's trial on charges of murdering two county policeman, said, "I observed Mr. Johnson with a blue steel revolver in his hand. He had his feel Positioned apart. He brought the gun up with both hands, kind of in a military grip and fried four distinct shots."

Roberts said he was standing inchess away from Swart. "I turned to the officer next to me and I rememver thinking, 'Why isn't this guy moving'? The I said, 'Leths get the hell out of here.'''

Roberts said he ran past Swart becacuse Swart was moving so slowly. "I didn't realize he was shot," Roberts said. "I was just trrying to get cover for Steven Roberts at the point.

"But when I looked back I saw that he had fallen over on his back and he was bleeding from the stomach and his mouth was foaming."

Roberts' testimony was the most damaging yet in three days of proscution testimony in the tense, emotional trial of the 16-year-old Johnson on charges of killing Swart and officer Albert M. Claggett IV.

One other key prosecution witness, Cpl. Paul Low, the man who captured Johnson seconds after the shooting, testified yesterday before the prosecution rested its case. Speaking in a low voice, obviously fighting to keep his emotions in check, Low said he ad been processing a prisioner of his own at about 2 a.m. when Swart and Claggett came into the station with Terrence and his brother, Melvin Johnson.

"Rusty (Claggett" and I were good friends so we talked for a minute," Low remembered. "I said, 'It looks like you're going to get some overtime.' He was upset because he was supposed to go stright home."

Low said he twice walked in and out of the processing room where Claggett and Swart had taken Johnson, then returned to the district commissionerhs window to finish some paperwork.

"I heard semthing that sounded like a firecracker or something," Low said. "I looked up then started back to my paperwork. The I heard four or five shots. I drew my weapon and took one or two steps forward. Then the (hall) door burst open and I saw a black male urnning toward me with his hands straight ahead of him his head up, holding a gun.

"He aran right toward me," Low continued. "It sounded like he was yelling of screaming something. I didn't want to take a chance on shooting because I knew there were police officers in the area behind him (Johnson).

"When he reached me I grabbed him and wrestled him down and we went into the wall together. I yelled for him to given up the gun and he yelled 'no'. The only thing I could think to do was hit him on the head with my gun."

Low said that even when Johnson was struck on the head he held onto the gun until Low took his foot and slammed it on Johnson's wrist, breaking the gun free.

Low then confirmed Roberts' earlier testimony that after Low had handcufed Johnson, Roberts hit the youth once in the stomach and kneed him, Low said, "below the waist."

Low said that after he and Roberts took th struggling Johnson back to the holding cell where his brother Melvin was, Low looked in the fingerprinting room and discovered Claggett, in his back, mortally wounded.

"I knew the medics were on the way so I tried to talk to him," Low Said. "I tried to tell him to fight it, just fight it. I yelled at him hoping he would hear me."

As he finished, Low put his head in his hands to try to regain his composure.The Packed courtroom was silent except for the quiet crying of Claggett's relatives. When the court recessed moments later, Low wept in the hall, overcom after having gone through a step-by-step reenactment for the jury.

But his testimony, combined with that of Roberts, left defense attorney, R. Kenneth Mundy, who had been buoued by Thursday's testimony which onfirmed much of the defense verision of story, disheartened.

"I think yesterday (Thursday) was a round for the defense and today was definitely a round for the prosecution," Mundy said after State's Attorney Arthur A. Marshall Jr., hand finished calling witnesses.

Before he rested his case, Marshall called FBI agent Richard A. Crum, dwho testified that the shot that killed Claggett was fired from "no more than two inches away."

Marshall called one surprise witness, officer Ray Evans, a black officer, who testified that he arrived at the Hyattsvile station moments after the shooting and found Terrence in the holding cell with Roberts and Low.

"He said to me, 'Help me, brother, don't let them do this to me,' " Evans said. "I hit him (with his fist) twice, in the face."

Mundy sdaid later he thought Marshall called Evans "to try and prove that even a black officer was outraged by the shootings."