An Alexandria Circuit Court judge yesterday ordered a man to serve the full 20 years to which he was sentenced for a 1974 second-degree murder, saying the man had violated his parole by sending a threatening letter to his mother.

Because of a letter he wrote just days before he was released on parole, Hugh Platt Price Jr., 34, of Alexandria, now must serve 10 years of a 20-year sentence that were originally suspended. The letter was not brought to the attention of federal officials until after Price was paroled and had returned to his mother's home.

He is currently serving a five-year federal sentence for the "threatening communication" sent while he was in the Mecklenburg Correctional Center in Boydton, Va. The 20-year sentence he was serving there was for the 1974 stabbing death of Jerome P. Lakenen of Alexandria.

The letter, addressed to his mother, Hattie Price, threatened to kill his two sisters, his aunt and his attorney.

One of his sisters, Mary Lou Price, an executive secretary at a government agency, was not present at yesterday's trial but read over the phone from a copy of the letter received in 1982: "I hate my sisters -- enough to kill them both. . . . Don't mention Grace [his aunt] to me again because I would like to cut her head off and leave her in a pool of blood."

Alexandria Circuit Court Judge Albert H. Grenadier ruled that Price was "dangerous and should be kept incarcerated."

Platt's defense attorney, James Clark, had argued that though the letter was "damning," his client clearly suffered from "psychological problems" and would be better off under strict supervised probation.

Clark said Price had lived with his mother and sister in Alexandria for the 10 days he was paroled in 1982 and "nothing happened." Only after he had moved back home did Mary Lou Price discover the letter, which her mother had said nothing about. She brought it to the attention of the FBI.

"From the letter, he clearly had no business on the street," his sister said. "But it is sad he has to go back to the Virginia prisons. I think there is a lot of violence in them."

After Price finishes his present five-year term in North Carolina's Butner corrections facility, a federal prison near Durham, for sending the threatening letter through the mails, he is scheduled to return to Mecklenburg.

Frank Tavenner, the Annandale attorney who defended Price in 1982, said yesterday that in his opinion Price meant no harm. "It the letter was just a childish endeavor to threaten his sister. They never got along anyway."

The only words Price uttered during the trial were to his attorney: "I'd like to appeal." The judge granted his request.