The 54-year-old woman was picking up litter along 44th Avenue in Hyattsville late one August morning when Osman Thomas Leekins Jr. walked up to her and, without warning, put his hands around her throat and tried to choke her.

Leekins was arrested two weeks later, on Sept. 12, 1995, and was charged with battery. Two days after that, his mother filed an emergency petition in Prince George's County District Court, asking a judge to involuntarily commit Leekins so he could undergo a psychiatric evaluation, according to court records.

The judge granted the request, and Leekins later pleaded guilty to battering the woman. As part of his sentence, he was given two years' probation and ordered to take medication to treat his mental illness, records show.

Police now think it was not the first time that Leekins had tried to strangle a woman along 44th Avenue. Shortly after midnight Sunday, police said, Leekins, 24, walked into the Prince George's County police station in Hyattsville, asked to see a homicide detective and told investigators that he had strangled 24-year-old Sunita P. Johnson after she got off a bus on Oct. 2, 1994, less than a year before the second incident.

Detectives said Leekins took them to the scene of the crime--a drainage ditch near 44th Avenue and Wells Parkway in University Park--and described the attack in enough detail that they quickly came to believe his story. He was charged with first-degree murder late Sunday and is being held in the Prince George's County jail.

Police said yesterday that Leekins voluntarily admitted to killing Johnson, who was five months pregnant, but that they were unaware at the time that Leekins had a history of mental illness. "He was very lucid, very alert and very methodical when he told the detectives his information," said police spokesman Royce D. Holloway.

But criminal law experts said that defense lawyers would almost certainly ask a judge to determine if Leekins was mentally competent to waive his right against self-incrimination when he made his statement to detectives.

"It does raise the question: Was his illness acting up? Was his whole confession a symptom of his mental illness?" said Richard Ortega, a supervisor with the Mental Hygiene Administration, a state agency that evaluates criminal defendants. "A defense attorney would be duty-bound to investigate whether or not his mental health played any role in his confession."

Leekins does not yet have a lawyer, court officials said yesterday. He will be assigned a public defender next week if he does not hire an attorney before then. He is being held without bond in jail and is scheduled to appear in court for a preliminary hearing Aug. 10.

Court records describing the nature of Leekins's mental illness--such as affidavits filed by his mother when she sought to have him involuntarily committed in 1995--were not publicly available yesterday. Leekins's mother, Barbara A. Leekins of Richmond, declined to be interviewed.

The woman whom Leekins tried to choke in 1995 said in an interview that she felt sorry for Leekins and his family after she learned from neighbors that he was mentally ill.

"That's the reason I didn't push too strongly to put him in jail," said the woman, who asked not to be identified. "After I heard all the stories about him being a mental case, I felt sorry for him, and I did not want to hurt his mother or sister."

The woman said she had never seen Leekins before he approached her as she was picking up litter outside one of her rental properties about 10:40 a.m. on Aug. 30, 1995. Suddenly, without warning, he grabbed her neck with both his hands, she said.

"I tried to fight him, but it was impossible because he was so strong," the woman said. "I screamed: 'He's a murderer! He's trying to kill me!' "

The woman said Leekins ran away when a passerby heard her screams and tried to help. She said she reported the crime in person at the Prince George's police station in Hyattsville but became frustrated when officers declined to press charges.

As a result, the woman consulted an attorney and filed a criminal complaint against Leekins, aided by neighbors who identified him.

"The police wouldn't cooperate at all," she said. "I don't know why they wouldn't help me. But I filed charges because I was worried somebody else might get hurt."

Police said yesterday that they were trying to locate her report but could not verify if they had investigated the case.

CAPTION: Police said they were unaware that Osman Thomas Leekins Jr. had a history of mental illness when he told them of his role in assaults.