A woman and her 6-year-old daughter apparently drowned Thursday evening in a lake in Prince William County, authorities said.

Grace Darlene Shields, 40, and Raina Alexandra Shields were found in Lake Montclair, near their home on Royal Crescent Court, county police said. Montclair is about 31 / 2 miles south of Dale City.

Neighbors had seen the pair, along with Grace Shields’s 9-year-old daughter, walking to the lake and standing on a dock early in the evening. Officer Mark Merriman, a police spokesman, said it was unclear whether the family had been swimming there or whether someone had fallen in.

About 8:30 p.m., a neighbor saw the 9-year-old screaming at the top of a walkway leading to the lake, police said. They said the child could not explain what had happened because she has a developmental disability and was unable to communicate with them.

The neighbor took the child back to the child’s home and waited for the rest of the family to return, police said. When they didn’t, she tried to find the mother on her own in the neighborhood and then called police.

Kristina Tkac, who was visiting her parents in the neighborhood, said it was her mother who spotted the girl, stayed with her during the evening and called police. She said her mother had seen the family, along with a dog, on the dock that evening.

About 11 p.m., police went to the area and searched the neighborhood on foot and by a helicopter from Fairfax County. A scuba team later found two bodies “close to the dock near the home,” according to police. The two were not wearing life jackets, Merriman said.

Neighbors said the family moved in about a month ago and didn’t know many people. Merriman said the father was out of the country.

The 9-year-old was placed with Child Protective Services until a family member could take care of her, police said.

Merriman said it was unclear how the mother and child ended up in the water.

“We can’t place them as being in the water,” he said. “We don’t know yet if someone fell off the dock and somebody went in or not. We don’t have witnesses. We are still trying to piece together what happened.”

Michael S. Rosenwald contributed to this report.