An attacker killed two young brothers and stabbed their 13-year-old sister in a South Side apartment early Thursday, authorities said.

A suspect was taken into custody within an hour of police finding the boys, ages 12 and 14, unresponsive in their home, said police spokeswoman Monique Bond.

Police released few details of the attack. Charges had not been filed against the suspect as of Thursday evening.

Bond would only say that the man is a "known acquaintance" of the family who had been identified by the 13-year-old. He is a registered sex offender, Bond said.

The girl, who was stabbed in the neck, is in stable condition at a hospital. She managed to flee the home and go for help to a nearby gas station, Bond said.

The 14-year-old boy died of multiple stab wounds, according to the medical examiner's office. Autopsy results for the 12-year-old were expected to be released Friday.

Police arrived at the residence about 3:30 a.m. Thursday, Bond said. The boys were pronounced dead at 4:45 a.m.

At the family's yellow brick apartment building on Thursday morning, neighbors watched as staff from the medical examiner's office wheeled gurneys containing body bags down a narrow walkway between structures.

One teenager gasped as she saw the two body bags brought out of the home.