A home is seen Thursday in Frederick County, Md., where a mother, father and infant boy were fatally shot a day earlier. (Lynh Bui/The Washington Post)

In a quiet, wooded cul-de-sac in southern Frederick County, a 5-year-old girl walked alone Wednesday night to a house next door. She told a neighbor that her mom, her dad and her baby brother were hurt inside her home.

But they weren’t just hurt. All three of them had been fatally shot in what the county Sheriff’s Office is investigating as a murder-suicide.

The incident has left the girl in the custody of state Child Protective Services and stunned the rural town of New Market.

Deputies went to Woods Court in the Lake Linganore development about 8:15 p.m. Wednesday in response to a 911 call from the neighbor who had spoken with the girl. In the girl’s home, they discovered Benyam Asefa, 40, Barbara Giomarelli, 42, and Samuel Asefa, 3 months, all dead of apparent gunshot wounds.

“There was an argument in the home, according to the child,” said Capt. Tim Clarke of the Sheriff’s Office. A handgun was found at the scene, but it was unclear who fired the shots, authorities said.

Law enforcement officials and Child Protective Services spent Thursday talking to the girl about what she had seen and heard, being careful not to further traumatize her, Clarke said. The Sheriff’s Office is not expected to release further information until authorities can talk to Giomarelli’s relatives, who were scheduled to arrive Friday from Italy.

Giomarelli, an Italian national, had worked at Maryland’s Institute for Marine and Environmental Technology in Baltimore on a visa for three years, said Alex Likowski, a spokesman for the University of Maryland at Baltimore. She left the job in February, he said.

Asefa, who had a PhD, was most recently employed as a clinical immunologist for Lovelace Respiratory Research Institute, working in support of the National Institute of Allergy and Infectious Diseases Integrated Research Facility in Frederick, according to the National Institutes of Health. He had stopped working there in September, according to the NIH statement.

Frederick County typically records one or two homicides a year, and news of the three deaths sent shock waves through the quiet Lake Linganore community.

Aimee McBride said her son goes to Deer Crossing Elementary School with the surviving child. McBride said she would often chat with Asefa and Giomarelli as parents dropped off and picked up children at the bus stop. They seemed like a nice, happy family, McBride said.

“He recently lost his job, and she had to go back to work,” McBride said. “They had the typical financial strains, but nothing to explain any of this.”

Samuel, at 3 months, was just starting to smile, McBride said, and his sister, who was normally shy, had begun blowing kisses to the neighbors. McBride said she has started a fund to support the girl.

Hannah Montgomery, who has lived in the community since 1991, said her son is in the same class as the girl. Montgomery and a neighbor drove to the cul-de-sac Thursday morning. A bouquet of orange flowers lay on the family’s porch.

“That’s the kind of stuff that happens in Baltimore and D.C.,” Montgomery said, just before the car drove away. “When something like this happens here, it’s a total shocker.”

Clarence Williams and Jennifer Jenkins contributed to this report.