An elderly woman was fatally shot and her grandniece was strangled and stabbed yesterday in a Northwest Washington home by an assailant who then set the house on fire to cover up the slayings, police said.

Firefighters discovered the bodies of the two women shortly before 5 p.m. after responding to a fire at 1513 Buchanan St. NW. The fire, which was extinguished in about 10 minutes, apparently began in the basement of the house, one of dozens of row houses in the quiet neighborhood.

Mabel Gordon, described by police as in her seventies, was found on the first floor of the two-story house. She had been shot twice and was taken to Washington Hospital Center, where she was pronounced dead.

Gordon's grandniece, Marcie Blackstock, was found lying in the basement of the house. Blackstock, described as being in her early twenties, had several stab wounds and an electrical cord wound tightly around her neck, police sources said.

Police said they had no motives or suspects.

The killings shocked neighbors, many of whom watched apprehensively from porches and walked up and down the crowded street as fire and police officials investigated.

"This is an extremely quiet neighborhood," said Laverne Stephens, who has lived across the street for 20 years. Stephens said she had known the younger victim for more than 10 years.

"She was a homebody," Stephens said. "Even as a youngster, I'd always see her sitting on the porch playing with a cat."

Neighbors said five persons lived at the home. They said the family was Jamaican, and that language barriers often kept sidewalk conversations with them brief.

"They seem to be very friendly people," said a neighbor who did not give her name. "They've never caused any trouble at all."

Mickey Perry, who lives about six houses from the burned home, said police rarely have been called to the area, which he described as a safe, working-class neighborhood.

"This is just a nice little area, always pretty quiet," Perry said as he watched firefighters work at the burned home. Like almost all of his neighbors standing outside their homes, Perry knew little of what had happened. "A fire wouldn't be surprising, but if those two women were killed by somebody, that would be very surprising for around here."