A convicted murderer and burglar awaiting sentencing in Pittsburgh was identified yesterday by police there as the prime suspect in the Alexandria killing of Time-Life Books employe Constance Mellon last October.

Pittsburgh homicide Detective Terry Hediger said in an interview that, based on statements and a weapon police have recovered, Frank Weston, 27, is the suspect.

Earlier this week two Alexandria detectives and an assistant prosecutor from the Northern Virginia city interviewed Weston at the Allegheny County, Pa., jail where he is in custody, law enforcement officials said.

Weston was convicted in April of murder and robbery charges in the slaying of a shoe store clerk in Pittsburgh that occurred later on Oct. 4, the same day as Mellon's death.

Hediger said yesterday that a small-caliber handgun, a Walther PK38, was recovered at the store and that police believe the weapon was used in both killings.

Alexandria chief prosecutor John E. Kloch declined comment yesterday, except to say that Pittsburgh and Alexandria officials have been cooperating on the case. The case will be presented to an Alexandria grand jury Monday, Kloch said.

The 36-year-old Mellon's body was found Oct. 7 after coworkers became concerned about her absence from work.

Hediger said yesterday the Pittsburgh murder and robbery occurred about 2 p.m., some time after the Mellon murder about 200 miles south in Alexandria.

According to an account in The Pittsburgh Press, trial testimony showed that Weston took the show store clerk to the basement, bound him and shot him once in the head. Weston told police he was "high on speed" and "half-drunk" at the time, according to court statements.

Three customers testified, however, that a man they identified as Weston had later waited on them in the store and that he appeared to be acting normally. atWeston, a Pittsburgh native, listed an address in the Hill District, a low-income area of the city, during the trial, according to the newspaper.

Weston was found guilty of the charges by Common Pleas Judge Henry R. Smith Jr. on April 6. He has since been in custody awaiting a separate hearing before Smith on whether the death penalty will be imposed in the murder.

Weston was implicated in the shoe store slaying after Pittsburgh police received an anonymous tip, according to The Press. He was arrested Dec. 4 by Pittsburgh detectives at a correctional facility in Yardville, N.J., on the outskirts of Trenton, where he was serving time on a parole violation.

Spokesman James Stabile of the New Jersey corrections department said yesterday that Weston originally was sentenced in July 1979 to a 10-year prison term for assault with intent to rob in connection with a purse-snatching incident. Weston was living in East Orange, N.J., at the time.

He was paroled from Yardville in March of last year, but failed to report to his parole officer in May and his parole was revoked, Stabile said.

He was subsequently stopped by a New Jersey state trooper for a traffic offense within days of the two October slayings and was returned to the Yardville facility Oct. 28, according to the spokesman.

A source familiar with the Mellon investigation said Thursday he understood that Weston had become a suspect in the case after he discussed her slaying with a cellmate at the Allegheny jail.