Twelve years ago, Angelo DeFeo, then a 19-year-old marine at home on leave, told Prince George's County police that he had stabbed his father to death during a fight at the family's Riverdale home. Last week, his mother, Donna DeFeo, now 50, saying she was haunted by the truth, confessed to California police that it was she, not Angelo, who had wielded the knife that killed her husband.

"She spits it right out: 'I killed my husband and my son took the blame. I want to clean the slate,' " said Jamie Skeeters, a homicide detective who was on duty in Oxnard, Calif., when Donna DeFeo appeared to clear her conscience and her son.

Angelo DeFeo was charged by police with murder after his father's death in September 1971, but a grand jury declined to indict him, police said, apparently because the killing had been an act of self-defense. The state's attorney's office in Prince George's is now considering whether to take any action against Donna DeFeo. A decision is expected today.

Donna DeFeo said on Wednesday in a telephone interview that she never asked her son, who now works at an aircraft parts plant in Ventura, Calif., to confess, but he insisted on doing so.. "My son wanted to do it to protect me," she said of Angelo, the oldest of her seven children.

"We went through years of agony," said Donna DeFeo, who now runs a house-cleaning business near Ventura. She said her sleep was plagued by nightmares of the killing and of what she described as a ghastly life of beatings and abuse she and her children had endured at the hands of her husband.

"It was frightening back in that time," she said. "There was absolutely no help for someone who suffered when a man was abusing his family. I know people heard me, heard the kids, knew they were being abused. But nobody said anything; the police couldn't do anything."

She said she never complained to police about her husband, but neighbors did. But, Donna DeFeo said, nothing happened. "Today, people are more aware of what is going on and are more willing to help," she said.

According to Detective Skeeters, Donna DeFeo told him that her husband came home drunk on Sept. 14, 1971, and started beating her up in the couple's bedroom. Her son, Angelo, was in the house and tried to intervene, but her husband turned on him and started strangling him, Skeeters said DeFeo told him.

Donna DeFeo said she tried to pull her husband off her son without success, Skeeters said. "She didn't know what to do. She went back and picked up a knife and stabbed him six times. I think it was three times in the back and three times in the chest." Her husband, Armando DeFeo, staggered across the room and collapsed against a wall, Skeeters said DeFeo told him.

Two years after her husband's death, Donna DeFeo moved her family to California where, she said, "We wanted to start a new life." Skeeters said family members told police that Angelo was shunned and called names by some family members. "It was hard to live with the secret," Skeeters said.

DeFeo said she planned to write about her experiences with her family and had written many pages when the suitcase she kept the manuscript in was stolen. "I was trying to get it off my mind," she explained. "I wanted to forget it. I wanted to hold my head up again."

But she said she could not forget it and neither could her son. "It seemed to be on his mind a lot, and mine too," she said. "I used to feel guilty. People would look at me--such a nice person. Like, all my friends--I wondered how they'd feel if they knew the truth."

Her son "would talk about it a lot," she said. "In fact, he'd talk about it to the point where we thought, why can't he ever forget it?" Although the grand jury found he had acted in self-defense, DeFeo said, her son Angelo "felt guilty . . . . He took it on his shoulders. Now he is relieved. I'm glad for him."

So, gradually, DeFeo said, she decided the time had come to set the record straight. "If I suddenly died, there would have been no idea what the truth would have been," she said. "I could never have cleared it."

Six of her seven children already knew the truth, she said. She said she told her four oldest children she had killed her husband immediately after the stabbing, and two of the younger children when they got older. By the time she turned herself in to the police, only her youngest son, 12-year-old Micky, had not been told, she said.

She said she believed her children were old enough now to live with the consequences, whatever they might be. "The youngest son is going to be 13," she said. "He's not a baby any more . . . . They are okay now. I feel if something happened to me, they can cope with it."

"At first I thought she was mad," said Detective Skeeters about DeFeo's confession. "We aren't very far away from a mental hospital and occasionally people come in and confess to things they didn't do." But she showed him the death certificate and he checked with Maryland authorities, Skeeters said. Everything fell into place.

Skeeters said the family described the story to him in a series of interviews. "Judging from the story she gave up, we didn't hold her," he said. "We felt out here the killing was justifiable. And she's sure not going to run away. We told her she may go to jail, but she said she wanted to clear the slate."

DeFeo said she did not tell Angelo that she was going to the police, because she was afraid he would stop her. Skeeters said the son continued for a long time to tell police that he had killed his father and maintain that his mother had no part in it.

"Even when we were interviewing him, he denied it his mother's account --until the last minute," Skeeters said. That came when detectives told Angelo DeFeo that they would give his mother a lie detector test, he said, and at that point Angelo DeFeo "knew all was lost. He broke down and cried."

A Ventura newspaper publicized DeFeo's confession in its Easter editions, and, Skeeters said, the family was worried the community would reject them once the news broke. "But people were very kind," he said. DeFeo said friends, neighbors and even people she had not met in a long time, have telephoned to give their support to her.

"Even though people have been supportive, in the grocery store people look at me," she said. "I hate to go to the bank. It's very hard on me. I just feel like everyone's whispering behind my back. Someone looks at me funny and I think they are talking about me. I worry sick about the kids."

As for herself, DeFeo said, "I can never marry again. I will never be able to marry again. I could never take that risk again."