A man accused of fatally stabbing his girlfriend's two young sons and sexually assaulting her daughter told police he was disciplining the boys over their chores and the situation escalated, police said Friday.

Brian Gilbert, 36, of Chicago has been charged with first-degree murder, attempted murder and sexual assault.

The bodies of the boys, ages 12 and 14, were found at their home early Thursday, around the time their mother was arriving home from her job at O'Hare International Airport. They had been stabbed repeatedly with a steak knife.

Deputy Police Chief Michael Shields said Gilbert decided to discipline the boys about cleaning up and he became angry and struck one of them. The 13-year-old girl tried to intervene and Gilbert then attacked her, Shields said.

The girl, who was stabbed in the neck, was in good condition at a hospital Friday.

Gilbert could face the death penalty if convicted of first-degree murder. He was to appear in court Friday afternoon. It was unclear if he had an attorney.

The mother told the Chicago Tribune that she became alarmed when she returned home and found Gilbert, whom she had been dating for only a few weeks, outside her apartment. He told her that her sons had gone to the store.

After he left, she went inside, only to find the police there.

"I saw my youngest slumped in the side of a closet in the living room," she said. "I saw my baby boy."

The Associated Press is not identifying the mother and sons in order to protect the identity of the daughter, the alleged victim of a sex crime.

Police spokeswoman Monique Bond had said earlier that the suspect was a registered sex offender.