A 1-year-old girl died Wednesday after what the police said was an assault at a Brooklyn apartment, and the case is being investigated as a homicide.

Officers arrived at the building in Crown Heights about 6:45 p.m. on Friday and found the girl with injuries to her head and fractured femurs, the police said Thursday. She was taken to a hospital in critical condition and later transferred to Maimonides Medical Center, where she was pronounced dead.

Officials on Thursday did not identify the child or her parents. No one has been charged in the death, the police said.

The child had been in the care of her parents when she was injured, Lieutenant John Russo said at a news conference. The mother told investigators that she left the apartment at 718 St. Marks Avenue for a short period on Friday and said she returned to find the girl hurt.

“From the very beginning, we were investigating this as a very, very serious assault,” Lt. Russo said. The girl’s parents also have a 2-year-old son, who was placed in foster care with relatives, Lt. Russo said.

Hospital workers noted that the child’s injuries were “very significant.”

The Administration for Children’s Services had previously opened a case on the family, according to the police.

The grey stone apartment building where the child was found is on a residential street just off the busy Nostrand Avenue thoroughfare.Sharrod Deloach, 35, who lives in the building, said he was at work on Friday evening and arrived home to a heavy police presence that lasted through the weekend. Usually the building is quiet, he said.

Cynthia McCarthy, 63, a home heath aide, said when she returned to the apartment building on Sunday, she also saw officers stationed outside.

“The police said it was a crime scene and it was something with a child,” Ms. McCarthy said. “I said, ‘Did the child pass away?’ And he said, ‘Something like that.’”

Hurubie Meko contributed reporting. Sheelagh McNeill contributed research.