The arrest 10 months ago of a Washington man attempting to smuggle $1 million worth of high-quality heroin into the United States was a major turning point in a federal investigation of a highly sophisticated heroin smuggling operation based here, informed sources said yesterday.

The investigation was being directed by Barry L. Leibowitz, an assistant U.S. attorney who was shot and wounded Tuesday morning outside the U.S. courthouse here where he works.

Authorities said that the shooting was a professional job and that they believe it was intended as a message for potential witnesses from the highly structured, Washington-based rign that they have been investigating. Police have traced more than 100 pounds of high-quality heroin worth millions of dollars to the ring.

Sources have said the quantity of heroin involved is the largest by far of any smuggling operation in the Washington area.

Leibowitz, who was shot once in the back, was released yesterday from George Washington University Hospital with the bullet still inside him. Doctors said it was lodged in fatty tissue just above the right hip and did not need to be removed.

Authorities said yesterday that there have been at least two known deaths of witnesses or potential witnesses since the investigation began about a year ago. One person was killed in Amsterdam, Holland, and another in New York City. FBI and Metropolitan Police investigators are now known to be looking at some recent narcotic-related deaths in the District of Columbia to see whether they have any connection to the investigation headed by Leibowitz.

The two known murders have occurred as a task force of Durg Enforcement Administration and Metropolitan Police lawmen have attempted to piece together the dimensions of the smuggling ring. The investigation is now said to be focused on about two dozen people, with more suspects likely as other tentacles of the organization are discovered.

According to informed sources, the

investigation into the drug ring was under way, but not sharply defined until U.S. customs officials arrested George Franklin Carter, 36, as he tried to bring about two pounds of heroin into Chicago's O'Hare International Airport last February.

Carter, who lived then in the 3400 block of 25th Street SE, was on a scheduled flight from Amsterdam to New York City that was diverted to Chicago because of heavy snow in the Northeast.

Customs officials had been alerted by their internal intelligence to watch for Carter.

Carter, who was dressed in a three piece business suit, looked like an average tourist when he went through customs, according to a customs of ficial.

But when Carter was searched officals at O'Hare found four cartons cigarette packs. Of those packs, 36 contained heroin that was 85 percent pure. Heroin sold on the streets of Washington generally is about 3 percent pure.

Customs officials also found $6,769 on Carter although he made out a customs declaration of less than ,5,000.

Carter, a former house painter and Vietnam veteran, had no previous convictions, according to his attorney, Oliver Spurlock.

Carter was sentenced on May 16 to 12 years in prison for the heroin smuggling. At the sentencing, U.S. District Court Judge Joel Flaum said the evidence showed that Carter "must have been a part of a major, sophisticated scheme" involving others.

Sources close to the investigation believe that Amsterdam, where Carter's flight originated, is a major transfer point in the heroin pipeline to the Washington organization.

The heroin shipments are believed to originate in Thailand or Turkey, the sources said.

They also said the heroin is commonly smuggled in cigarette packs.

Leibowitz and an official of the joint Durg. Enforcement Administration-Metropolitan Police Department task force investigating the ring visited Amsterdam around Labor Day, sources said.

The heroin allegedly has been flown at various times into Chicago and then transported here for distribution throughout the United States, according to sources.

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

The success of the ring now under investigation is measured not so much by the number of people believed to be involved as by the amount of heroin linked to it, investigators said.

Leibowitz refused to comment yesterday on any aspect of the case or his shooting. U.S. marshals have been rushed from the hospital yesterday to an undisclosed location.

Leibowitz was shot when a car with two men inside approached him and the man on the passenger side rolled down his window and, holding a .22 caliber gun in his right hand, fired from about six feet.

The first shot ripped apart the prosecutor's tie, just below his neck, and the second struck him in the back as he turned away from the would-be killer.

While some lawmen investigating the attempted murder are known to believe the work was that of professional killers, other lawmen yesterday expressed surprise that the would-be assassins used a .22 caliber pistol.

Had the gunman used a .38 or .45 or even a .32 caliber, which makes only slightly more noise than a .22, "we'd have been going to a funerl," said one law enforcement official.

Authorities believe that a silencer was used, largely because none of the peope in the area heard any gunshots. With a silencer, officials said, shots from a pistol as small as a .22 would make no more noise than that of a solft drink bottle being opened.

Some officials are convinced that, because the killers seemed to know exactly when Leibowitz would know how to escape swiftly through a tangle of one-way streets, they knew what they were doing.

According to these theorists, the killers wanted would-be witnesses to know that if they would attack a prosecutor next to the courthouse, they would find and kill any potential witnesses anywhere else.