Two children were killed and two were injured at a home on Cherry Bend Drive in Germantown, Md. (Sarah L. Voisin/The Washington Post)

The 911 caller spotted a car with an open door and a knife on the ground outside a neighbor’s townhouse. Officers who pulled up to the Germantown community Friday morning knew the caller also reported seeing blood. They grabbed the keys from the car and went inside the home.

What they found, police said, was chaotic and devastating: two toddlers slain in an upstairs bedroom, two of their siblings badly injured and the children’s 28-year-old mother running out the back door. She was quickly captured and late Friday was being questioned as a “person of interest” in the case, according to Capt. Jim Daly, a Montgomery County police spokesman.

A second woman who was in the townhouse, described as a family acquaintance about 21 or 22 years old, had relatively minor injuries to her neck and was also termed a person of interest. No charges had been filed by Friday evening.

“We found a very horrific and bloody crime scene,” Daly said.

A 1-year-old boy and a 2-year-old girl were killed, police said. The injured children, a 5-year-old girl and an 8-year-old boy, were hospitalized in stable condition.

“This is a tragic situation,” Montgomery State’s Attorney John McCarthy said at the townhouse.

Initial indications were that the slain children had been stabbed, and detectives recovered a knife that they think was used in the attack. Police were awaiting results of autopsies to determine how the children died.

Police did not immediately release the names of those in the house. Neighbors said the family had moved in about eight months ago.

The townhouse — along Cherry Bend Drive — is about 13 miles north of the Capital Beltway. The story that detectives were trying to piece together began at least the night before, when other officers were called to the townhouse but didn’t go inside.

That time, about 9:45 p.m., a 911 caller reported seeing a child alone in a car. As the man stood outside talking to the dispatcher, it was clear that he suddenly was having a conversation with someone else. The man could be heard saying something to the effect that he was making the situation his business because there was an unattended child, according to a law enforcement official familiar with a recording of the call.

Two women had come out of the townhouse, confronted the man and taken the child inside, police said.

Officers who went to the townhouse knocked on the door, but there was no answer and they didn’t see or hear anything suspicious, Daly said. One police commander said they spent more than 40 minutes at the scene. But the officers didn’t find enough probable cause to force their way inside, he said.

The officers called the county’s Children’s Protective Services agency, which said it would follow up Friday morning, police said. Then they left. “The officers had no choice but to clear the scene,” Daly said.

A spokeswoman for Montgomery’s Health and Human Services Department would not say whether Children’s Protective Services has had any interactions with the children, citing privacy laws.

At least twice during the night, neighbors heard noises and disturbances coming from the townhouse, but nothing that alarmed them enough to call 911, police said.

About 7:15 a.m. Friday, Elisa Aloras, a neighbor, was walking her dog, Tobi, and spotted a kitchen knife on the sidewalk, she said in an interview. She didn’t see blood or anything else that made her suspicious, and kept walking.

Thirty minutes later, when she was driving her daughter to school, she saw an open car door and a child’s blue coat on the ground next to it. She didn’t call the police, a decision she regretted after learning what had happened.

“Now I say, ‘Oh, my God,’ ” she said. “I’m very upset because I didn’t call.”

About 9:30 a.m., a 911 caller reported having “noticed a vehicle with the door open and a knife laying outside the vehicle,” police said in a statement.

After the arriving officers grabbed the keys from the car, they approached the townhouse door.

“Montgomery County police!” they yelled, according to two neighbors.

Once inside, officers spotted the 28-year-old mother with a child — either holding or near the youngster (reports weren’t immediately clear). She bolted out the back door, but officers chased her and quickly caught her.

“They tackled her,” said a neighbor who requested anonymity to protect her privacy.

The woman who tried to flee was taken into custody and questioned Friday. The second woman was taken to a hospital for treatment of her injuries.

Police did not provide details about what might have sparked the violence, but according to Police Chief J. Thomas Manger, “Right now, we believe that we’ve got everybody involved.”

Magda Jean-Louis, Bill Turque and Donna St. George contributed to this report.

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