Three years ago, Louise Chandler, a 58-year-old housewife, stood in the doorway of her small duplex in Southeast Washington and watched a stranger watch her.

She saw the man, parked in front of her home, scribble something on a piece of paper, wait for about 30 minutes, then drive slowly down the the block, park in front of an apartment building, and enter, carrying a briefcase.

What Chandler saw that day, and a homicide detective's perseverance, culminated in the murder conviction Wednesday of an Adelphi insurance salesman who was found to have stabbed and bludgeoned to death a book store cashier living in the apartment building the stranger had entered on Oct. 25, 1979.

Chandler's testimony about her chance observation, law enforcement sources say, was vital to placing the salesman, Joseph Henry Burrows, 27, in the neighborhood at the time of the murder of Willie Bernice Lampkin.

In the weeks after the slaying, several incidents connected Burrows and the victim, and Detective Jeff Greene became convinced that Burrows was the killer. But the evidence submitted to the grand jury was not sufficient to obtain an indictment. He had not yet met Chandler. And he had decided not to give up.

Greene has declined to comment on the case. According to sources familiar with the case he was haunted by the image of Lampkin's 9-year-old daughter coming home from school and discovering her mother's body. He also was troubled by the knowledge that the girl and her 10-year-old sister are now orphans, living with relatives in southern Virginia.

According to officials familiar with the case, the events leading to the D.C. Superior Court conviction of Burrows unfolded in this way:

Burrows was arrested originally six days after the slaying. Investigators could show that he had been to the victim's home days before the killing to deliver a $5,000 check to Lampkin, the proceeds from a life insurance policy on her husband, who had died a month before.

The investigators could also show that he returned to see Lampkin several times after that, but found her away from home.

Witnesses would say that Burrows also had visited her at her job.

There was also evidence that an hour before Lampkin's body was discovered, Burrows had deposited in his bank account a $3,000 check issued to Lampkin from a second insurance policy on her husband.

There was no proof, however, that Lampkin had ever gotten the second check.

And there was no one who could place Burrows at the victim's home on the day of death.

A D.C. Superior Court judge, ruling at a preliminary hearing in 1979, found that there was not enough evidence to establish probable cause that Burrows killed Lampkin. A further grand jury investigation ended with no indictment.

The case continued to gnaw at Greene, whose 14 years as a D.C. homicide detective told him Burrows was the killer. From time to time he would thumb through the case jacket, looking for something new.

He discovered one new clue in this way about about 18 months after the slaying when he noticed that there was no record in the file that the fingerprint department had examined fingerprints on the $3,000 check. He and Assistant U.S. Attorney Steven Gordon asked a police fingerprint expert to make a comparison.

The examiner said that a thumbprint on the front of the check was indeed Lampkin's. The prosecutor and detective were elated. Now there was proof that Lampkin had the check at some point, and that Burrows had gotten the check back from her. But they believed they still had a shaky case. They could not show when Burrows got the check from her.

Greene recanvassed the neighborhood, looking once more for anyone who might have seen or heard something the day of the murder. A neighbor recalled that she had heard that someone in the neighborhood had seen something odd that day. The neighbor suggested the detective talk to Louise Chandler, who had lived in the neighborhood for 26 years.

Chandler, meeting with Greene last year, described seeing the stranger parked in front of her house and then go into Lampkin's apartment building around the time that detectives believed the murder occurred. She gave a general description of him and described his car, but could not make a positive identification of Burrows.

Still, prosecutors correctly felt that the weight of the circumstantial evidence was now enough to obtain an indictment.

Then, two weeks before the trial, as Greene pored over the case file in Gordon's office, he noticed a small slip of white paper that he had seen before but had given no significance. It was one note among many Burrows had written.

The note said: "Saw me at 8:12." Written on the paper was an address on B Street SE. The address was Louise Chandler's. That was the note, Greene and Gordon believed, that Chandler had seen the stranger write that morning in October 1979.

During the week-long trial, Gordon weaved together the circumstantial case: showing that Burrows needed money, showing that he had cashed the $3,000 check, showing that Willie Bernice Lampkin had at some point had the check in her hand, and showing, through Chandler's testimony, that Burrows had gone to Lampkin's apartment building the morning of the murder.

James Joiner, the defense attorney, argued that there was no evidence that his client was the killer. The bits and pieces of evidence were enough, however, to convince the jury that the insurance salesman had killed the widow.

The jurors deliberated less than three hours before convicting Burrows of armed robbery and murder. Chief Judge H. Carl Moultrie I ordered Burrows held pending sentencing Dec. 17. He faces a mandatory sentence of 20 years to life in prison.