An assistant U.S. attorney who is leading an investigation into an elaborate, Washington-based international heroin smuggling ring was shot and wounded yesterday morning by one of two men who drove past the prosecutor as he was walking into the federal courthouse here.

Police said two shots were fired at point-blank range at the prosecutor, Barry L. Leibowitz. One bullet narrowly missed Leibowitz's neck, slicing through the knot in his necktie. The other bullet stuck him above his right hip and lodged in his lower back, law enforcement officials said.

Between 20 and 25 investigators from four law enforcement agencies began a massive hunt for the two men, who sped away from the shooting scene in a 1973 or 1975 light green or yellow Buick.

Leibowitz was reported in fair condition last night at George Washington University Hospital. Hospital authorities said they may not remove the bullet in Leibowitz's back that he may be released sometime today or tomorrow.

Sources close to the investigation said they had no immediate suspects but were assuming that Leibowitz may have been shot by someome connected with the heroin smuggling operation.

The location where Leibowtz was shot contains the highest concentration of law enforcement officials in the Washington area. He was shot immediately across the street from D.C. police headquarters, adjacent to a federal courthouse with dozens of judges, prosecutors, marshals and other security personnel, and six blocks from the FBI headquarters.

Leibowitz, 31, was described yesterday by colleagues and defense attorneys alike as an extraordinarily tenacious and hard-driving prosecutor with an abrasive personality. In the last year he has won convictions against 34 persons in two major drug cases, including one that led to the federal confiscation of Sylvester's Restaurant in downtown Washington after a jury found it was a front for a cocaine distribution operation.

Two years ago, Leibowitz personally arrested a teen-aged pickpocket on a Metrobus as he was riding home from work. Yesterday, he pursued his assailants for about 10 feet before realizing that he had been shot, according to investigatiors.

The heroin smuggling investigation has been going on for the last nine months to a year, according to sources close to the investigation. They said a grand jury has been hearing testimony for the last six months and that scores of witnesses have been called.

Investigators have traced more than 100 pounds of exceptionally high-quality heroin to the ting. The heroin-which one federal investigator said was worth millions of dollars in street value-allegedly has been flown at various times into Chicago and then transported here for distribution throughout the United States, according to the sources. It was not immediately known, however, where the heroin shipments originated.

Authorities said the heroin ring under investigation is highly structured, with a distinct hierarchy including a number of "lieutenants." This contrasts sharply, the authorities said, with the smaller-scale, loosely organized rings usually responsible for the distribution of heroin in this area.

The quantity of heroin involved is by far the largest amount ever involved in a single District of Columbia drug case, federal and local authorities said. They said the largest previous amount was five pounds in a case last week.

One source familiar with the investigation said that indictments and arrests are expected soon in the case, possibly early in the new year.

According to source close to the shooting investigation, Leibowitz called his office on the fourth floor of the federal courthouse yesterday morning and said that he was going to be late for work. After going to the cleaners, he took the subway to work and got off at the 4th Street NW exit of the Judiciary Square station.

The rposecutor walked south along the east side of the D.C. police headquarters, crossed C Street and walked into the parking lot adjacent to the courthouse.

Leibowitz was walking on a sloping driveway leading to an underground courthouse garage about 10:05 a.m. when his assailants, described as being in their 30s, drove up along his left side.

Something apparently caught the prosecutor's eye, the source said, and he looked to the side and spotted a man in the front passenger seat with a gun pointing out the window at him. The prosecutor told investigators that one of the men may have said something to him and then the gunman fired the shots.

As the first bullet pierced his tie, slicing it off at the knot, Leibowitz turned again, according to the sources, and then was hit above his right hip by the second shot. Leibowitz chased his assailants' car for a moment and then felt the pain of the gunshot.

According to one source, Leibowitz began to yell, "Police! Get the police!" Another assistant U.S. attorney who was in the parking lot to move his car ran into the courthouse for help while a third prosecutor, also in the vicinity, went to assist Leibowitz.

Federal Protective Service officer Raymond Jackson got to the scene moments later. He found Leibowiz leaning against a dark blue Imperial and helped him to a stone wall leading to the underground courthouse garage.

"He was holding his black," Jackson said. "He was not bleeding too much or anything, just sitting there and the [police] came and picked him up.

"He was very calm. He didn't say anything."

Police found a spent, small-caliber pistol shell on the pavement near the shooting scene. By last night police had confirmed that a 22-caliber pistol was used in the shooting and they believe a silencer was attached to the pistol.

Even though the attempted murder failed, authorities theorized last night that professionals attempted the killing.

"Even the pros make mistakes," one source said, nothing that many professional killings are executed with 22-caliber weapons. "Leibowitz is a very lucky man-he just turned at the right time."

Authorities theorized that the shooting was deliberately planned to take place next to the courthouse and police headquarters in order to deter witnesses in the drug case.

"They were out to deliver a message," one source said, "and that was that if they'll go after a DA, they'll sure get any witnesses up on 14th Street, or anywhere else in the city." Some of the suspects in the drug investigation "are facing some very heavy time-they've got nothing to lose," the source said.

Dr. Paul W. Coughlin, the surgical resident at GWU Hospital who treated Leibowitz, said the prosecutor was very lucky because "fortunately he turned quickly when he realized he was being shot at."

The surgeon said the bullet that lodged in the prosecutor's back can be felt just under the skin in the fatty tissue. Leibowitz is six feet tall and weighs about 180 pounds.

"It is not essential that we take [the bullet] out," Coughlin said.

U.S. marshals threw up a tight security ring around Leibowitz's hospital room and sources said the protection would continue for the indefinite future.

U.S. Attorney Earl J. Silbert said at a late afternoon press conference, held in a drizzle outside the courthouse, that he had spoken with Leibowitz in the hospital and that the prosecutor was "alert and, all things considered, is doing well."

Silbert said he had spoken with U.S. Attroney General Griffin B. Bell and Deputy Attorney General Benjamin Civiletti about the incident and both were "particularly concerned about a shooting of a law enforcement officer engaged in his official duties of carrying out" the laws.

But Silbert decline to answer any question about the shooting investigation or the drug investigation that Leibowitz is working on. He said the shooting investigation is being handled by his offece, D.C. police, the FBI and the U.S. Drug Enforcement Administration.

Police authorities issued a lookout yesterday afternoon for a man, about 38 years old and wearing a dark overcoat. The auto was described as a 1973 or 1975 yellow or light green Buick Electra.

The Federal Protective Service patrols the buildings and the grounds of the U.S. courthouse here, located at Constitution Avenue and John Marshall Place NW, a short distance west of the Capitol. An officer patrols the Third Street parking lot where Leibowitz was shot, but the officer was on the Constitution Avenue side of the six-story building at the time of the shooting, according to FPS Deputy Chief Robert Washington.

Security is always tight at the courthouse and visitors without credentials must pass through a metal detector before they are allowed to enter. The shooting of Leibowitz is almost certain to intensify security preparations for the Jan. 9 trial of three Cubans charged in connected with the 1976 Embassy Row bombing death of former Chilean ambassador Orlando Letelier. The lives of a prosecutor and the judge in that case were threatened in August.

Leobowitz, who is somewhat stocky and has brown hair and a full mustache, is a graduate of the Newark (N.J.) College of Engineering and the American University School of Law, where he was an editor of the school law review.

He has worked for six years as a prosecutor, steadily advancing in the U.S. attorney's office here from handling misdemeanors, grand jury matters, appeals and felonies in D.C. Superior Court to prosecuting major crimes in the last two years.

"He's a very aggressive prosecutor," said one of his associates, Assistant U.S. Attorney Charles J. Harkins. "I wouldn't want him prosecuting me."

John Keats, a defense attorney who opposed Leibowitz in a major drug case last year, said that "any type of major narcotics investigation-that was his meat." Keats said that Leibowitz was the type of prosecutor who would bring witnessess back two and three times for interviews to make certain that he had covered every detail possible.