U.S. District Court Judge John H. Wood Jr., known for the stern sentences he imposed on narcotics smugglers, was shot to death in the parking lot of a fashionable condominium complex here today.

Wood, 63, was struck in the back by a single shot as he stood at the door of his automobile. He was pronounced dead on arrival at a nearby hospital.

A spokesman for the Administrative Office of U.S. Courts said Judge Wood's murder was the first known slaying of a federal judge.

Attorney General Griffin B. Bell, expressing shock and anger about the killing, dispatched one of the FBI's top criminal investigators to look into the matter and placed four other federal judges in the area under special protection.

"We cannot have a country where judges are assassinated or killed," Bell told a news conference. "If our society has become so lawless that you can just kill federal judges, or any judge for that matter, we haven't got a system of law. It's really a dark time in this country."

Bell, himself, a former federal judge, said, "We don't intend to leave a stone unturned." He said that 30 FBI agents already were in San Antonio and that another 30 were on the way to work on the killing.

Bell was joined by FBI Director William H. Webster, another former federal judge, in announcing that James O. Ingraham, deputy assistant director in the FBI's criminal division, will be in charge of investigating the killing of Wood.

Bell, who described the incident as "such a dastardly act in a free country," said U.S. marshals would be assigned to protect federal judges in the San Antonio area.

He and Webster said the FBI was investigating the possibility that the shooting was related to narcotics dealing and two previous shootings at federal law enforcement officials.

Judge Wood, who presided over trials in Del Rio and El Paso, as well as here, was believed to have one of the heaviest narcotics-related caseloads in the nation.

Defense attorneys often referred to him as "Maximum John," but law clerks who served under him said that the title was unfair because the judge frequently agonized over "taking away a man's freedom."

One of his most publicized cases was that of bigtime professional gambler Jimmy Chagra, who is scheduled to go on trial on July 23 on charges of conspiracy to possess, import and distribute cocaine and marijuana. Federal investigators have accused the defendant of being one of the major smugglers of South American cocaine and marijuana into the United States.

Lee Chagra, Jimmy's eldest brother and a colorful El Paso defense attorney, was shot to death in his office on Dec. 23.

Joe Chagra, another brother who is serving as Jimmy Chagra's defense counsel, had sought to have Wood removed from the case.

In a pretrial hearing last month, defense attorneys argued that the judge was biased and prejudiced.

Wood overruled their motion, but granted a change of venue, moving the trial from Midland to Austin. The ruling was appealed to the 5th U.S. Circuit Court of Appeals, but Wood's decision was affirmed.

The murder of the judge took place 14 blocks from the scene of a Nov. 22 ambush in which two gunmen fired 15 rounds of .30-caliber bullets into the automobile of Assistant U.S. Attorney James Kerr as he left home for work, injuring him slightly.

Kerr, who specialized in narcotics prosecutions, tried many of his most important cases before Wood.

The judge was placed under guard by federal marshall following the Kerr incident, but asked the marshals to discontinue their escort service shortly before Christmas.

"He felt the had a job to do and couldn't do it with all of his movements fettered," said Ron Johnson, who has served as a law clerk for Wood since last August.

Other Associates said that threats to the life of the judge were nothing new. Howard Newton, an attorney who clerked for Wood in 1974, recalled that the judge was guarded for a time after federal agents reported a threat against his life in El Paso.

After the November ambush, Kerr was kept under cover until the returned to federal court in El Paso in January to prosecute a narcotics-smuggling suspect defended by Joe Chagra with Judge Wood presiding.

The defendant, after being convicted of conspiring to smuggle nine tons of marijuana and cocaine into the United States, gave a television interview in which he discussed his smuggling experiences.

The interview was played back in the El Paso courtroom just before Wood sentenced the smuggler to a 30-year prison term.

Details of Jduge Wood's murder were sparse. Federal agents kept reporters away from the immediate scene and maintained tight wraps on information. First reports were that the judge had been shot by a rifleman. He was struck in the small of the back and the bullet lodged near the upper part of his chest.

San Antonio homicide investigators reported that they had found two witnesses to the shooting. It was not revealed whether they had seen the assassin; they were placed under cover by federal officials.

Wood had been a leading San Antonio civil trial attorney before his elevation to the federal bench in January 1971.

He was recommended to President Nixon by Sen. John G. Tower (R.-Tex). Wood had turned aside earlier overtures, and Tower reportedly had difficulty persuading to accept the judgeship.

Tower said today that it was with "deep sorrow and outrage" that he learned of "the assassination of an old and valued friend."

Wood was from a poineer Texas family. His great-great-grandfather was with Gen. Sam Houston at the Texas victory of San Jacinto. His grandfather was a popular Democratic sheriff of Bexar County. CAPTION:

Picture 1, Medical attendants work over Judge John H. Wood Jr. in San Antonio. AP; Picture 2, JUDGE JOHN H. WOOD JR. . . . received earlier threats