Police found the body of a 2-week-old girl yesterday in a plastic trash bag in the Shenandoah River and charged her mother, who had reported the child missing from her pickup truck in a parking lot in Winchester, Va., with first-degree murder.

The mother, Kathleen Householder, 21, of Ripon, W.Va., was held last night at the Jefferson County Jail in Charles Town, W.Va.

She had told police Jan. 3 that the baby, Lindsey Blake, was taken from the unlocked truck while she was in an IGA grocery store. Winchester Police Chief Allen Barley said yesterday that after a lie detector test on Monday she told authorities to look for the infant in the river. The murder charge was filed last night after she was questioned by West Virginia police.

"As far as we can tell, that was all fabricated," said Barley. "We had our doubts from the beginning." Police would not discuss the lie detector test results, or how the baby had died.

Householder initially told police she drove from her West Virginia home to Winchester with the baby, Lindsey Blake, and her son, Dustin, 2, to collect her paycheck from the Winchester Medical Center, where she worked as a secretary. She had dressed the infant in a pink outfit, with a shirt that read "baby."

Police said the baby's father, Daniel, was working on a construction project near Washington.

After stopping at the medical center, Householder said, she drove to a nearby IGA to cash the check and buy cigarettes. She said she took her son into the store, but left the baby in the truck because she was asleep and it was cold outside. She said the baby was in a car seat and covered by a blanket. When she returned to the truck between three and 10 minutes later, she said, the baby was gone.

A sobbing Householder made public appeals that night for the return of her child. "Just don't hurt her," she said. "I don't want any trouble from anybody. I just want her back."

The National Missing Child Search Society later offered a $15,000 reward for information leading to the location of the child.

Yesterday's search of the river began at 12:30 p.m., with four police divers, one helicopter and search dogs. Virginia and West Virginia troopers and Clarke County, Va., sheriff's deputies participated in the search.

The baby's body was discovered by the dogs about 3:40 p.m. in 18 inches of water at the river's edge in Berryville, just off Rte. 622, and about a tenth of a mile south of Rte. 50 in Clarke County, said Sgt. Jack Turner of the Virginia State Police. He said the infant appeared to be wearing the pink outfit her mother had described.

The body was taken to the Enders Funeral Home in Berryville, Turner said, and an autopsy will be conducted.

In a telephone interview from her West Virginia home with The Washington Post on Sunday, Householder said she had not slept well since her child disappeared and was in touch with the police and the National Center for Missing and Exploited Children.