A 14-year-old Southeast Washington boy will appear in D.C. Family Court today to be formally charged with the sexual assault and beating death last week of his 4-year-old sister, police said yesterday.

The youth was arrested Saturday night by the D.C. police department's special victims unit in connection with the death of Angel Fleming.

Police said the beating occurred Tuesday between 9:30 and 10:30 p.m. inside the two-story rowhouse in the 1100 block of Chicago Street SE where the youth lived with his grandmother, 8-year-old brother, 5-year-old sister and Angel. The grandmother, Viola Robinson, 60, had tucked the two girls into bed before making her nightly trip to pick up her husband from work, police said.

When Robinson returned about 45 minutes later, she went upstairs to the bedroom where all four children slept, and the 14-year-old told her that Angel was vomiting and having difficulty breathing, according to police. They said that about midnight, Robinson drove her granddaughter to the emergency room of Children's Hospital, where the girl underwent surgery for internal bleeding and a damaged liver and spleen.

Police said that in addition to those injuries, there was evidence that Angel had been sexually assaulted.

D.C. social workers removed the three other children, including the suspect, from Robinson's home Wednesday and placed them in foster care, police said.

Angel died shortly after 3 p.m. Thursday, police said. The D.C. medical examiner's office ruled Friday that the cause of death was multiple blunt force trauma. Her 14-year-old brother was arrested at 11:40 p.m. Saturday at the home of his foster parents, police said. The 8-year-old and 5-year-old remain in foster care.

The suspect, whose name was not released because he is a juvenile, faces charges of felony murder and first-degree child sexual abuse. District law forbids trying anyone under age 16 as an adult, police said. If convicted, the youth could remain in custody until age 21.

Police said he initially told investigators during questioning that Angel fell down a flight of 13 stairs in the house. He repeated that story before admitting that he struck the girl several times and sexually assaulted her, police said.

Angel and her 5-year-old sister had moved in with their grandmother only a few weeks before the incident, police said. They said the children's mother is in jail in Prince George's County after being picked up recently on a warrant. The two boys have lived with Robinson since they were infants, according to police.

Robinson's neighbors said they were shocked by the girl's death and the arrest of her brother.

"I saw all the police up here," said Glovenia Harrison, who lives nearby. "That is so sad. They all used to play in the street."

Neighbor Charles Williams, who said Robinson drove his grandchildren to school, was nearly speechless when he learned about the arrest.

"My God," Williams said. "Oh, man. I can't talk right now."

Neighbor Marie Johnson described the 14-year-old as a "nice kid" and said he was "nice and respectful" when he came to her house to visit her grandchildren.

"He never disrespected me," said Johnson, 72. "He acted like a perfect gentleman when he came here."

Johnson said Angel and her sister used to jump rope outside their house and ride their bicycles up and down the street.

"She was just a baby," Johnson said. "I don't . . . believe he did it. But I don't know. Children change, especially when they get in their teens."

Staff writer Del Quentin Wilber contributed to this report.