An unidentified assailant killed a 12-year-old girl and raped her two young sisters in their home in the northern Mexico border city of Ciudad Juarez.
The prosecutor's office in Chihuahua state says girl died of strangulation.
The office did not give the younger girls' ages, but local news media said they were 10 and 11, reports AP.
The office said Friday the assailant is being sought, but did not say if he had been identified. The attack occurred Wednesday in a low-income neighbourhood on the city's west side, near a desert area where the bodies of numerous young women were found dumped in the 1990s.
Human rights officials in Mexico say number of women and girls killed in the country since then has almost doubled.
Luis Raul Gonzalez Perez, president of Mexico's National Human Rights Commission, said Friday that the number of women killed per year in Mexico has risen from 1,519 in 1990 to 2,813 in 2016.