MEMPHIS, Tenn. (AP) _ A ``mentally challenged'' man found sobbing and kneeling in tall grass by a neighbor early Thursday was arrested in the shotgun slayings of four people believed to be his adoptive family, police said.

``He said, `What have I done? I killed them. They're going to put me away for a long time,''' said the neighbor, Raymond Neal, who was accompanied by an officer when he approached the man he knows only as ``Timmy.''

The 21-year-old man, whose name was not released, was found behind the house where the victims were found when police responded to a burglar alarm.

The victims were identified as Larry Prink, 49; his wife, Barbara, 48; their adopted daughter, Heidi McCarthy, 41; and Barbara Prink's mother, Dorothy Lynch, 79. All were shot at close range, apparently by a 12-gauge shotgun found nearby, police said.

The suspect was believed to be the Prinks' adopted son, said police Lt. Richard True. The man was not charged, but was being held as police gathered evidence.

True declined to elaborate on his description of the suspect as ``mentally challenged'' or discuss a motive.

State mental health officials were evaluating the three women found unharmed in the home and a fourth woman who had gotten out earlier. At least some of them have mental deficiencies that leave them ``unable to take care of themselves,'' True said.

Two of the women lived with the Prinks in a foster home arrangement, according to Tom Sullivan, assistant commissioner for the state Department of Mental Health and Mental Retardation. Sullivan said his agency had no reports of problems at the Prinks' home.

Neal said he had little contact with his neighbors and did not know anything about the living arrangements.

``They were just different people,'' he said.