A member of the Prince George’s County police comforts a mourner on Brooke Jane Drive in Clinton, Md. (Bonnie Jo Mount/The Washington Post)

Three girls under 10 were found dead in a house Friday morning in Maryland when the mother of one of the children arrived home and came upon the horrific scene.

Prince George’s County police said late Friday that they had arrested a 24-year-old man in the case.

They identified him as Antonio Williams of the 6400 block of Brooke Jane Drive in Clinton, Md. That is in the same block as the home in which the children were found. Police said additional details would be provided Saturday.

Detectives were working to determine the relationships of the children, and said they thought that the violence was “specific” to the house in Clinton and that the community at large was not at risk.

After the mother and others called 911 about 7:30 a.m., police arrived at the home in the 6400 block of Brooke Jane Drive and met family members who reported that three juveniles were inside with trauma to the body.

Prince George’s County police are investigating the deaths of three juveniles found by a relative on the morning of Aug. 18 at a home in the 6400 block of Brooke Jane Drive. (WUSA)

A fourth child, a 2-year-old, was inside, alive and unharmed, according to a law enforcement official familiar with the case.

Assistant Police Chief Hector Velez said the entire community is mourning the “three beautiful young children.”

“We all feel this loss,” Velez said. “We want to assure the community that the Prince George’s County Police Department will work tirelessly to find the person or persons responsible for these children’s deaths.”

It’s unclear who was caring for the children, who was in the house, and how long the girls had been dead.

Police were also working to determine where all of the children lived and how many people lived at the home.

“We have a lot of questions ourselves,” Deputy Chief Sammy Patel said.

Police would not identify the girls or give their ages as of late Friday, saying they were still working to notify family members.

Homicide prosecutors also were on the scene Friday morning, pledging to “leave no stone unturned,” Prince George’s County State’s Attorney Angela Alsobrooks said.

“We woke up this morning, as hard as it is, to yet another nightmare,” Alsobrooks said. “We are absolutely heartbroken.”

Authorities launched a massive investigation, with police tape stretching across several blocks of the quiet neighborhood of single-family homes.

Counselors were also on hand for investigators and other first responders — many of whom have children of their own — working at the traumatic scene, said Jennifer Donelan, a spokeswoman for Prince George’s County police.

“This is one of the most difficult that our officers have arrived on,” Donelan said.

By late morning, residents in the neighborhood were learning about the deaths. Neighbor James Pinkney called it “earthshaking news.”

“This is horrific,” Pinkney said. Pinkney said he would stay home from work Friday, worried about leaving his 13-year-old daughter home alone as police searched for suspects.

“My heart is heavy,” said a choked up Pinkney. “How could you live with yourself committing such an evil act?”

Keith L. Alexander, Magda Jean-Louis, Perry Stein, Martin Weil, Justin Wm. Moyer and Peter Hermann contributed to this report.