A police officer in Chicago shot and killed two people on Saturday (local time) after responding to a domestic disturbance on the city's west side.
The Chicago Police Department said the officers "were confronted by a combative subject" after the callout at 4:25am.
Quintonio LeGrier, a 19-year-old student, and Bettie Jones, a 55-year-old mother, were pronounced dead at hospital at around 5am.
Both victims were black, the Cook County medical examiner's office says. The officer's race or identity has not yet been disclosed.
The Chicago Police Department is already under fire after the release of a video showing white officer Jason Van Dyke shooting black teenager Laquan McDonald 16 times in 2014, prompting a federal civil rights investigation.
Van Dyke faces six counts of first-degree murder and one charge of official misconduct. Officials are investigating patterns of racial bias in the use of force by Chicago police as well as how the department handles the accusations against officers.
Quintonio's father, Antonio, told the Chicago Sun-Times his son had been a "little agitated" after he chose not to go to a family holiday gathering. He said he called police when, at around 4:15am, his son tried to bust his locked bedroom door open, yelling, "You're not going to scare me."
Ms Jones, who lived a floor below, said she had seen Quintonio outside with a baseball bat.
Antonio told the paper he heard Ms Jones yell "whoa, whoa, whoa!" when police arrived and heard gunshots as he came down from the second floor to see his son and Ms Jones lying in the foyer.
Chicago's Independent Police Review Authority is investigating the shooting, but said it was too early to comment, the Associated Press reports.