A South Portland man was charged with murder Friday in the 1983 stabbing death of 23-year-old Judith Flagg.

A grand jury in Kennebec County earlier in the day had indicted 49-year-old Thomas Mitchell Jr. in connection with the slaying. When he was indicted, Mitchell was already serving time at the Maine State Prison on unrelated charges, police said.

The development in one of the state’s most notorious unsolved homicides was announced by Maine State Police at a news conference early Friday night in Augusta.

Flagg’s body was discovered in her Watson Heights Road home Jan. 6, 1983, when her husband returned from a double shift at work. Police said Flagg’s 1-year-old son was near the body when it was discovered. The child was unharmed.

Flagg had been stabbed several times in the chest and head. At the time, police said there was no indication that Flagg knew her attacker. The knife was not found.

Mitchell, born in February 1957, was 25 at the time of the slaying, though it was not immediately clear if he had been living in or near Fayette at the time.

State Police on Friday night were not elaborating on Mitchell’s criminal record, any known links he may have had with Flagg, or what led to the indictment after nearly a quarter-century.

“Any new information will come through the court system,” said Maine Department of Public Safety spokesman Stephen McCausland.

Police have investigated several tips in the case for the past 23 years, but until Friday afternoon, the case remained unsolved.

Theodore Flagg, Judy’s widower who now lives in Livermore, was informed by State Police recently that there had been developments in the case.

A woman who answered the telephone at his home Friday night said he was still absorbing the news. Theodore Flagg was not ready to answer questions about the slaying or the long-awaited charges filed on Friday.

“He’s refraining from comment right now,” the woman said. “It’s been a very long and emotional day.”

On the day of the slaying, Theodore Flagg said goodbye to his wife about 5:15 a.m. as he set off for a 16-hour shift at work, according to police accounts. When he returned home at about 11 p.m., his wife was dead on the floor, the 1-year-old boy nearby.