Kathleen Householder, who is charged with murdering the newborn baby daughter she initially reported as missing, told police under questioning that she had hit the baby in the head with a rock, a state police trooper testified at a court hearing here today.

Cpl. Leonard E. Lanham, who administered a lie detector test to Householder, said the woman told him that she had become frustrated because her 2-week-old daughter Lindsay, and her son Dustin, 2, "were fussing" in the cab of the family pickup truck, so she picked up a baseball-sized rock from her yard and threw it at the baby.

"It was my fault . . . . I didn't mean to," Lanham quoted Householder as saying.

Lindsay began to "quiver and shake," and Householder put the infant in a brown plastic trash bag and threw it into the nearby Shenandoah River, Lanham said the woman told him.

"She believed she did see [the child] move a time or two in the bag," before she threw it into the river, Lanham testified.

Magistrate E.W. Day ordered that Householder, 21, who was charged Jan. 21 with first-degree murder in the infant's death, remain in custody without bond pending grand jury action in the case.

Householder, who reported the child missing Jan. 3, told police at the time she had left Lindsay on the seat of her unlocked pickup truck while she stopped at a grocery store in Winchester, Va., and that when she returned to the truck the infant was gone. Police mounted a massive search for the child, a $15,000 reward was offered for finding her, and Householder made numerous television appearances to issue pleas for the child's safe return. "Just don't hurt her," she had sobbed.

She wept again today as a Northern Virginia medical examiner described how the child's 8-pound, 7-ounce body, wearing a pink and white shirt and a single bootie was found at a site in the river in Virginia about 20 miles from Householder's Rippon, W.Va., home. The infant was apparently killed by a blow to the head, but the body also showed some signs of asphyxiation, the medical examiner said.

Lanham, one of three witnesses called by prosecuting attorney Michael Thompson, said he administered the lie detector test to Householder Jan. 21 after she had led police to a spot on the river where they found the body.

Lanham said Householder told him at one point during the test that the baby had toppled from a bassinet and struck her head on the floor. She said then that she panicked, put the baby in a trash bag and drove to the river, Lanham testified.

Later, Lanham said, Householder told him Lindsay had fallen from her arms and hit her head as she was carrying the child down some steps. When he told her the medical examiner would be able to tell how the baby died, Lanham testified, Householder started crying and told him about throwing the rock.

Defense attorney Steve M. Askin challenged much of the prosecution testimony, demanding to know why investigators believed one version of Householder's story and not the others, and suggesting that police "selectively chose" the worst version.

Under cross-examination, Askin asked State Police Cpl. Gregory A. Stevens if at any time during the investigation Householder had admitted she had -- according to the statutory definition of first-degree murder -- "feloniously, willfully, maliciously and deliberately" killed her daughter. "No sir, she did not," Stevens replied.

Householder, who was in court wearing blue prison garb, appeared drawn but composed through most of the daylong hearing at the Magistrate Court Building here. Her husband, Daniel, 21, sat quietly throughout the proceedings, once whispering in her ear.

The grand jury will consider Householder's case in April, according to Thompson. He said she could receive a life sentence if convicted on the charge.