Hoping to prevent the release from prison of a drug offender who has served a lengthy sentence, the Manhattan district attorney, Robert Morgenthau, is attempting to link the inmate to the 1988 murder of a rookie police officer.

The inmate, John McCaskell, of Queens, has served 15 years since his arrest in 1990 for carrying four ounces of cocaine. Two years later, he was sentenced to 25 years to life. Now he is seeking release under the revised Rockefeller drug laws. He is one of an estimated 440 prisoners eligible for relief under the revised drug laws, which were rewritten by state legislators late last year to allow more lenient prison sentences.

McCaskell’s application for resentencing has proved to be more tangled and sensitive than most, as prosecutors attempt to connect him to the killing of the policeman, Edward Byrne, who, while guarding the Queens home of a witness, was shot in the head five times. The killers were members of a violent drug gang run by the infamous kingpin, Howard “Pappy” Mason, now serving out a life sentence for Byrne’s murder and other crimes. Since the last years of the Reagan administration, a memorial fund in the name of the slain officer has been used by the Justice Department to distribute hundreds of millions of dollars in grants for local law enforcement.

Yesterday, in a virtually empty courtroom, prosecutors argued that McCaskell was street lieutenant in Mason’s drug gang. They have alleged in court papers that he hid the weapon used to kill Byrne. The problem for the district attorney’s office, however, is that they have presented no evidence to suggest that McCaskell disposed of the murder weapon or had any role in Byrne’s murder.

The basis for the allegation is a confidential informant working for the Police Department inside Mason’s drug empire. The informant was initially charged with a variety of drug-related crimes, records show, but agreed to cooperate with police in obtaining evidence that could lead to the arrests of co-workers.

According to police records made public before the McCaskell hearing began, the confidential informant told detectives three years after the Byrne murder that McCaskell had once told him he stashed the gun used to kill the police officer.

The police records also show that the same informant incriminated himself in a separate murder, of a cab driver used to make drug deliveries, Soeradj Bhikharie.

While in prison on the drug charges, the informant allegedly told police that McCaskell, who at the time went by the street alias “Born,” ordered fellow drug dealers to murder Bhikarie because the cabbie could become a witness against him in court.

The attorney handling McCaskell’s resentencing application, Margaret Ratner-Kunstler – wife of the famed defense attorney William Kunstler – said it was unfair for prosecutors to use “the innuendo of a crime” without presenting any evidence to support it.

Ms. Ratner-Kunstler also pointed out that the Byrne murder was considered one of the most thoroughly investigated homicides in New York history and that McCaskell had never been charged in connection with it.

In about 500 internal reports filled out by police detectives in the case, McCaskell’s name was mentioned in only two reports, when it was cited by the confidential informant who implicated himself and others in the yet-unsolved murder of the taxi driver.

Ms. Ratner, who serves as director of the William Kunstler Fund for Racial Justice, a nonprofit organization that has been active in protesting the strict Rockefeller drug laws, said that under the revised sentencing guidelines Mc-Caskell could have been released from prison years ago.

The man’s mother, Mayella McCaskell, and his sister, Nicole, were the only two visitors watching the hearing in Manhattan Criminal Court yesterday.

“Fifteen years is a long time,” Mc-Caskell’s mother said.

The state Supreme Court justice who will rule on McCaskell’s resentencing, Marcy Kahn, has so far been wary of any attempt to connect McCaskell to the Byrne murder, calling the link “rigorously disputed.”

The judge herself became a matter of dispute at the hearing yesterday after Ms. Ratner-Kunstler attempted to call as a witness a state senator who helped write the measure easing the Rockefeller drug laws, Thomas Duane, a Democrat of Chelsea.

During her election campaign in 1994, Mr. Duane worked for Judge Kahn as a campaign volunteer. Prosecutors said yesterday the senator’s role in the hearing created “the appearance of impropriety” and requested that he submit an affidavit instead of testifying in person. Judge Kahn said she needed more time to consider the matter.

“It seems kind of silly,” Mr. Duane said yesterday, in a telephone interview, of prosecutors’ concerns that his relationship to the judge could tarnish any testimony.

“Who the judge is has nothing to do with my expertise on this issue,” Mr. Duane said.