A convicted international drug smuggler pleaded guilty yesterday to conspiring to murder a U.S. prosecutor who was shot here in 1978 while he was investigating the smuggler and others who were importing $30 million worth of high-grade heroin to Washington.

The plea by Robert L. Stuckey, 42, is the first step in developing a case against others involved in the shooting of former Assistant U.S. Attorney Barry L. Leibowitz. The prosecutor was slightly injured as he walked across a parking lot at the federal courthouse Dec. 20, 1978.

At his drug trial last year, Stuckey testified that Linwood Gray was the person who actually shot Leibowitz. Gray was the alleged kingpin of the heroin operation, but was acquitted of drug charges in the case while convicted of tax evasion.

In a telephone interview yesterday, Leibowitz said, "I felt all along that the people responsible for shooting me were the people I was investigating."

In entering his guilty plea, Stuckey said that he not only conspired with Gray, 35, but also with Joseph F. Wilson Sr., 45, who also was acquitted of drug charges in the heroin scheme.

Principal assistant U.S. Attorney Carl S. Rauh, who negotiated the guilty plea with Stuckey, described it as "a major step forward in bringing before the bar of justice all those responsible for the shooting...."

No charges have been filed against either Gray or Wilson in connection with the Leibowitz shooting.

At his drug trial last October, Stuckey testified that Gray told him he had shot Leibowitz and that Gray said that if "it hadn't been for Wilson's good driving" the two of them would have been apprehended at the courthouse.

Assistant U.S. Attorney Robert R. Chapman told U.S. District Court Judge Howard Corcoran yesterday that if the government had taken the murder conspiracy charge against Stuckey to trial, it could prove that Gray told Stuckey several times that "Leibowitz should be killed."

Chapman told Corcoran that the government would show that Stuckey, at Gray's direction, made an anonymous telephone call to Leibowitz on the D.C. jail where a key figure in the drug investigation was imprisoned.

Chapman also said the government could prove that if Leibowitz had gone to the jail, "he would have been killed by Gray and Wilson." Instead of going to the jail, however, Leibowitz had the inmate, George Francis Carter, brought to the federal courthouse -- where Carter said he had no knowledge of the telephone call and did not want to talk to Leibowitz.

According to the government's evidence, the day before Leibowitz was shot in Washington, Stuckey went to a location near the federal courthouse and attempted to help Gray and Wilson start a car that they planned to use as a get-away vehicle.

After the shooting, Chapman said he could prove Gray told Stuckey to retrieve the .22-caliber automatic pistol and silencer from the car, which had been abandoned, and to then set the car on fire. Stuckey got the gun, which he later turned over to federal law enforcement officials, and then set fire to the car in the garage of the Hecht Co. a short distance from the courthouse.

Stuckey, a tall, thin man with a shaved head and a long, stringy goatee, stood beside Chapman as the prosecutor recounted the details of the case. Judge Corcoran then asked Stuckey, "Is that the way it happened."

"Ah, uh, yes sir," Stuckey responded.

Corcoran has not set a date for Stuckey's sentencing. Stuckey faces a sentence of up to life in prison.

Stuckey already is serving a 10-year prison term for his convictions on 45 narcotics counts in connection with the heroin ring.

Corcoran sealed Stuckey's plea bargain agreement with the government. Stuckey has been described by the government as a lieutenant in the drug group that smuggled heroin concealed in Marlboro cigarette packs from Amsterdam to the United States.

The government accepted Stuckey's guilty plea to a formal statement of the charge against him, instead of taking the case to a grand jury for its consideration. In such cases, defendants usually have agreed to cooperate with the government in connection with further prosecutions, and may in exchange receive protective services from law enforcement officials and recommendations from prosecutors to the court for less than maximum sentences.

At yesterday's hearing, Stuckey said that he was particularly concerned that the government "live up" to a portion of the plea agreement that relates to his teen-age son. But Stuckey did not elaborate further.