The police on Saturday were investigating the gruesome killing of a woman in a Brooklyn public housing development that left another woman seriously injured.
An ax covered in blood was recovered from a trash compactor in the housing development, the New York Police Department said, though it was not immediately clear if it was used in the attacks.
A 21-year-old woman had called for an Uber outside the housing complex, the Bushwick Houses on Flushing Avenue, around 1:30 a.m. on Saturday, the police said. The driver saw that she was bleeding heavily from cuts to her head and stomach and called 911.
While she was being taken to Elmhurst Hospital Center, the woman told emergency medical workers that she had been attacked by her former boyfriend and that her daughter and a female friend were still in her eighth-floor apartment, the police said.
Police officers arriving at the apartment discovered a grisly scene: a partially decapitated woman on the floor, with some of her fingers severed and multiple stab wounds. The police identified the woman as 20-year-old Savannah Rivera.
A 4-year-old girl was found unharmed in a rear bedroom, the police said. The child was taken to Woodhull Medical and Mental Health Center for evaluation. The wounded woman was in serious condition on Saturday night, officials said.
The police were questioning a person of interest, said Deputy Chief Michael Kemper, the commanding officer of Brooklyn North Detectives. He said that what happened was a “terrible, vicious attack” that left both victims with deep lacerations.
The person of interest was a 24-year-old man who previously had a romantic relationship with the wounded woman, a police official said, speaking on the condition of anonymity because he was not authorized to talk publicly about the case.
Chief Kemper told reporters at a briefing that both women knew the person being questioned by the police but he declined to elaborate.
Ms. Rivera was the mother of a 4-year-old boy and lived at a low-rise public housing development in Williamsburg, not far from the Bushwick Houses.
In an interview at her family’s apartment, which was decorated for Easter with colored plastic eggs hanging from the vestibule ceiling, her older sister, Tatiana Cabrera, remembered Ms. Rivera as “a very quiet girl” who “liked to party.”
About a year ago, Ms. Cabrera said, her sister began hanging around the wounded woman.
“Me and her had problems for years,” Ms. Cabrera said, referring to the wounded woman, who had not been publicly identified. “I told Savannah that her downfall would be this girl. I told her to stop hanging out with her.”
Ms. Rivera’s relatives said that her mother died in 2009 and that Ms. Rivera and two of her sisters were raised by their grandmother.
The grandmother, Elsa Collazo, said that when she learned of the killing, “I wanted to die.”
Ms. Rivera’s son sat on a couch, happily playing a video game. His fifth birthday, his grandmother said, will be on April 30.
“When he is old enough, I will tell him about his mother,” Ms. Collazo said. “She was a good mother.”
Outside the red brick building where the attack took place, residents reacted with a mix of shock and resignation.
Rochelle McGee, a resident of another building in the Bushwick Houses who works with a local anti-violence intervention group, said she learned about the killing on Facebook.
Renee Hamilton, who lives on the 16th floor of the building where the attack took place, recognized the wounded woman in a photograph.
“I ride the elevator with her in the morning when she takes her little daughter to school,” Ms. Hamilton said. “I’m going to pray for her.”