A 25-year-old theft suspect hanged himself from the bars of his cell at the Prince George's County Jail in Upper Marlboro yesterday. He had previously attempted suicide while being held at the jail and been diagnosed as suicidal at a state mental hospital.

The prisoner, Fred O'Neal, jailed under $100 bond awaiting trial, was found about 5:40 p.m. suspended by his shirt, which had been torn into strips and knotted together. He was pronounced dead on arrival at Southern Maryland Hospital.

O'Neal had attempted to hang himself once before since his arrival at the jail March 19, according to a county Corrections Department spokesman. He then was examined for two weeks at Spring Grove State Hospital at Catonsville, Md., and diagnosed as suicidal.

O'Neal's hanging was the second in a Prince George's jail facility in less than two months. The first, in which a 21-year-old prisoner died, was also ruled a suicide, although the county NAACP protested that assessment.

After O'Neal returned from the hospital, he was confined by himself in the Upper Marlboro jail's mental observation section, where prisoners are frequently checked by guards, according to corrections spokesman Jim O'Neal.

A guard making his regular rounds had seen O'Neal unharmed not more than 10 minutes before he was found hanged yesterday and was never more than 50 feet from the cell, the spokesman said.

O'Neal had no cellmate primarily because "he had proven himself from his initial entry here to be a behavioral problem," O'Neal said. Other prisoners in the six adjacent cells in mental observation area were confined in pairs in their sleeping quarters, he said.

Spokesman O'Neil said the cells where the prisoners sleep are separated from each other by solid steel walls, and prisoners invterviewed after the hanging said they heard no sounds coming from O'Neal's cell, which is at the end of the cell row.

O'Neal had been served dinner an hour earlier and had eaten it, O'Neal said. He was also administered a tranquilizer shortly after dinner by a medical techician, according to O'Neill.

The drug, which the spokesman would not identify, was prescribed during O'Neal stay at Spring Grove Hospital from April 2 to 15. The prisoner was sent there under a court order obtained by the Corrections Department April 1.

A staff psychologist at the hosptial had determined that O'Neal suffered from "some kind of mental attitudinal problem. He was determined to be suicidal," O'Neill said

Prince George's police investigating O'Neal's death would not immediately give the details of the theft charge on which he was awaiting trial. O'Neal, who lived in the 3900 block of East Capitol Street NE, had been jailed at the Upper Marlboro facility at least twice previously in the last four months on similar charges, O'Neal said. Police would not give details of those charges last night, either.

"Overall, people look at the severity of the crime and the fact that a guy's bail is set at $100 and say, 'Well, why would a person want to kill himself over this?'" O'Neill said. "But it's been shown that the severity of a crime for which a person is jailed can't really be used as a determinant in profiling a suicidal personality."

On March 4, 21-year-old Curtis A. Pelt of Palmer Park, Md., was found hanging by his shoelaces from a cell crossbar at a lockup in Hyattsville following his arrest for allegedly failing to pay a $25 dinner tab. The Prince George's NAACP protested Pelt's death and called for an investigation, but police concluded their inquiry after a state medical examiner ruled the death a suicide.