Walter O. Johnson may not have come to Washington to kill Metro Transit Police Officer Marlon F. Morales, but he did, prosecutors believe, come to the city three years ago to kill someone.

Fresh out of federal prison, where he had served time for bank robbery, Johnson most likely wanted revenge on a Northwest Washington woman who had turned him in back in 1989, prosecutors revealed in court papers this week.

Defense attorneys for Johnson, who have maintained that he was not even in Washington the night of the shooting, say the theory is baseless and point out that prosecutors offer no evidence of any plot.

Johnson, 36, is to be sentenced today in D.C. Superior Court, two months after a jury convicted him in the June 2001 slaying of Morales. Johnson was the first person convicted under a law that mandates life in prison for murder of a law enforcement officer.

Described by authorities as a recalcitrant and dangerous inmate, Johnson once beat up Mafia boss John Gotti in a fight at a prison in Marion, Ill., according to prison records. He was repeatedly transferred from one prison to another for disciplinary problems, and now he could end up back in one of the country's most secure lockups.

The court papers, submitted ahead of the sentencing, offer new information about what the government believes brought Johnson from Philadelphia to Northwest Washington and, in particular, to the U Street-Cardozo Metro station.

It was there, on June 10, 2001, that Johnson shot and mortally wounded Morales, 32, after the rookie officer stopped him in what seemed like a routine case of fare evasion.

In that deadly encounter, Morales may have unwittingly saved the life of Leona Williams, who had turned Johnson in 12 years earlier and who lived just a few blocks from the U Street station, prosecutors said.

Prosecutors believe that Johnson was out for revenge when he boarded a Greyhound bus from Philadelphia to Washington the day of the transit officer's shooting.

They traced events to the fall of 1989, soon after Johnson had absconded from a correctional facility and robbed a Philadelphia area bank of nearly $100,000. Prosecutors said Johnson came to Washington shortly after the heist and went to see Williams, a family friend, at her home.

But she was not there, and her son, wary of the stranger, offered only to let him leave his bag for safekeeping, prosecutors said. Authorities said about $88,000 was found in the bag -- along with a rifle that Johnson had stolen from his parents and used in the robbery.

When Johnson returned, pounding on her door in the early hours of the morning, Williams was puzzled and called his parents, she said yesterday in an interview. They told her about the bank robbery and urged her to call the police, Williams said.

She did, and Johnson was quickly taken into custody. As he left, Johnson turned to Williams, according to prosecutors' account.

"Aunt Leona, why did you do this?" he asked her.

"Because your daddy told me to," she replied.

Johnson was given a 10-year prison term for the bank robbery. He was locked up from October 1989 until May 2001 for the holdup and other offenses.

During his trial this spring, Judge Ann O'Regan Keary did not permit prosecutors to introduce any evidence of Johnson's 1989 trip to Washington. But for sentencing, both sides have more leeway to delve into his past.

Sitting in the living room of her house yesterday, Williams said she does not know what Johnson was doing in her neighborhood the night Morales was shot. For a time when he was growing up, she said, his family lived nearby. She wants to think that he wasn't coming after her.

"Maybe I'm wrong," she said. "I don't think Walter would have harmed me. But I don't know. After I called his father and called the police, who knows?"

In an excerpt of the written arguments the defense has submitted to the judge, lawyers Renee P. Raymond and Yvonne Williams of the D.C. Public Defender Service said the prosecution's contention that Johnson was in the area to kill Williams "is complete government conjecture and speculation."

By the time Johnson was confronted by Morales at the Metro station, the ex-convict was already in trouble. Morales just wanted to run a routine check on Johnson after he tried to exit the Metro system without paying.

For Johnson, however, the minor infraction could have quickly escalated, prosecutors said. He had never reported to his probation officer in Philadelphia, and a warrant had been issued for his arrest. Furthermore, prosecutors say, Johnson was carrying a gun that he had used to shoot a man a few days earlier in Philadelphia.

So when Morales asked for identification, Johnson went for his gun, Assistant U.S. Attorneys June M. Jeffries and David J. Gorman wrote in the sentencing memorandum filed this week.

"For Johnson, the only way out was to eliminate Off. Morales," the prosecutors said in the filing.

Shot in the head and stripped of his gun and extra ammunition, Morales, a father of three, lay on the station floor. Three days later, after suffering a stroke, he died at Washington Hospital Center.

It was the second time in the 28-year history of the transit police that one of its officers had been killed. Johnson eluded the initial police dragnet and returned to Philadelphia, prosecutors said. He was arrested there four days after the shooting, when he was stopped for a traffic violation and found to be carrying Morales's gun.

Jennifer Morales , the slain officer's widow, said the possibility that her husband may have prevented another killing does nothing to ease her pain. "My husband is still dead," she said in a telephone interview. "It's not that I want somebody else to die in his place, but it's hard," she said, her voice trailing off into silence.

Walter O. Johnson, left, came to Washington in 2001 to kill a woman who turned him in to police in 1989, prosecutors say. Washington will be sentenced today for killing Marlon Morales, a Metro police officer, right.