Montgomery County police dug up the backyard of a home in Suitland Thursday in an unsuccessful attempt to find the bodies of Katherine and Sheila Lyon, young sisters who disappeared from their home in Kensington seven years ago.

Two detectives spent more than three hours shoveling a dozen "test holes" behind a vacant brick rambler at 5816 Suitland Rd. with the permission of the owners of the house.

The dwelling was formerly owned and occupied by Raymond R. Mileski, 46, now serving a 40-year sentence in the Maryland Penitentiary in Baltimore for killing his wife and 17-year-old son, who were shot to death in the basement of the house in November 1977.

Montgomery police said they focused their attention on the Suitland house, which is in neighboring Prince George's County, after getting leads from penitentiary inmates.

The holes dug Thursday were sunk in areas where the ground appeared to have been disturbed, according to police. Each was several inches deep, sufficient to reach undisturbed, compacted soil.

At one point, the digging stopped when the detectives unearthed several small bones, one of which resembled the arm bone of a child, according to Det. James King. But Dr. Lawrence Angel, an anthropologist at the Smithsonian Institution, said after examining the bones that all were non-human and one was the bone of a large bird.

The Lyon sisters--Katherine, 10, and Sheila, 12--have not been seen since March 25, 1975, when they walked from their home on Plyers Mill Road to the Wheaton Plaza Shopping Center.

Katherine, who would now be 18, and Sheila, who would be 20, left their home about 11 a.m. The girls had less than $4 with them, according to their parents, John and Mary Lyon. Their mother said she expected them home no later than 4 p.m.

The last known sighting of the pair came about 3 p.m. when their 15-year-old brother Jay saw them eating pizza at a small restaurant in the shopping center, which was crowded with children out of school for the spring vacation.

Montgomery police officials say the search for the sisters is the longest and most extensive investigation ever conducted by the county force. More than 2,000 leads have been investigated, some with the aid of tracking dogs or helicopters. In once instance, a California psychic was consulted, according to King, who is in charge of the investigation. Among the theories that have been explored are that the girls were murdered, that they ran away or that they were kidnaped.

After unsuccessfully probing the Suitland backyard on Thursday, the detectives said they were careful to smoothe over the earth where they had dug the holes.