The 18-year-old son of a Loudoun County police lieutenant was arrested yesterday and charged in the shooting deaths last month of a thoroughbred horse and a prized cow in a rural area near Middleburg, Va.

The suspect, Thomas Lee Swain, is charged with maiming a chestnut quarter horse owned by Patricia Harriman, wife the former U.S. ambassador W. Averell Harriman, and with maiming a purebred Red Angus cow owned by Peter Goltra, whose estate is across the road from the Harriman property.

The shootings last month in the heart of the Loudoun hunt country shocked and frightened residents who own valuable horses and other livestock.

Investigators said someone using a high-powered rifle fired a hollow-point bullet into the neck of the horse, named Wingrove, then fired a bullet through the liver of the pregnant cow, named Elia. The cow was killed instantly; the horse lingered for four days under the care of a team of veterinarians before it died.

Swain, son of 12-year county police veteran Lt. Edward T. Swain, was arrested in Middleburg yesterday by Virginia State Trooper J. S. Holsinger, who headed the investigation. Swain was indicted on the maiming charges by a Loudoun Circuit Court jury on Monday.

Trooper Holsinger said his investigation is continuing. There have been other shootings of animals reported in the area, but he declined to discuss those cases.

A hearing for Swain, who attends Loundoun County High School, was scheduled for Dec. 18. Conviction of maiming animals carries a maximum sentence of 10 years in prison for each offense, officials said.