In a case involving the police killing of a teenager, a jury in federal court here has convicted two fired Houston policemen of planting a gun to make the incident look like self-defense and of then lying to a federal grand jury.

But the jury found the former officer who said that he did the shooting not guilty of depriving the youth of his civil rights by killing him, a charge that could have resulted in life in prison. And it acquitted a fired police supervisor of all charges against him.

The case is one of several involving allegations of civil rights violations by local law officers that are being pursued by the U.S. attorney here, Tony Canales.

The verdict comes on the heels of Justice Department attempts to overturn a federal judge's ruling that gave 10-year suspended sentences to three former Houston policemen convicted on civil rights charges that they beat up a young Mexican-American and dumped him in a bayou, resulting in his death by drowning. The Justice Department argues that suspended senctences aren't legal in cases that carry the possibility of life in prison.

In the current case, a 17-year-old Shreveport, La., youth, Randy Webster, was shot after he allegedly stole a van in the early hours of Feb. 8, 1977, and led police on a high-speed chase before wrecking the vehicle, Then-Patrolman Danny Howard Mays, 32, told police, a Harris County grand jury and a federal grand jury that he shot Webster after the youth lunged at him with a gun.

However, a taxi driver who had joined in the police chase testified that Webster was trying to get out of the van with his hands in the air when Mays dragges him out, threw him to ground and shot him in the head at close range.

Another former Houston officer, William E. Byrd, testified that he supplied the long-barrelled .22-caliber pistol used as the "throw-down" gun and which the U.S. attorney's office had traced to the police property room. Byrd had pleaded guilty to a lesser charge.

In addition to the charges against Mays, the government had charged former policeman Norval Wayne Holloway with complicity in the alleged civil rights violation, conspiring to cover up the fact that Webster was unarmed, and lying to the federal grand jury. The jury found him guilty only of cover-up and perjury.

Paul Dillon, 38, a former police supervisor who arrived on the scene immediately after the shooting, was found not guilty of complicity, cover up and perjury. Dillon was the only one of the three former officers charged who took the stand in his own defense.

Several jurors said the gun proved crucial to the jury's decision. One, Patricia McGuire Sullivan, explained: "It was the key. No one wanted to convict the young man, but we couldn't get around the throwdown gun."

The dead youth's father, John R. Webster, complained, "those twelve people on the jury are telling the Houston policemen that they have a free hand, 'Just go ahead and kill anyone you want to."

The youth's mother, Billie Webster, said, "We felt like the government had a strong case, presented it well and did a fine job. I don't understand how the jury could convict the two officers on the cover-up and perjury charges and not on the violation of Randy's civil rights."