A couple returned home from the cinema to find a naked man had beaten their grandparents to death and was raping their two-year-old daughter.
Touch Chea, 71, and his wife, Sorn Sreap, 73, were found dead in Anchorage, Alaska, on Saturday night from significant blunt force injuries.
Police said both Sreap and the toddler were raped.
Officers arrested Jerry Andrew Active, 24, as he fled the east Anchorage homicide scene wearing just his boxer shorts.
The registered sex offender was later charged with first and second-degree murder, sexual assault and burglary.
Active was on parole for an almost identical attack he committed four years ago.
In 2009 he entered a home in Togiak and sexually assaulted an 11-year-old girl while the family slept. He then attacked three members of the family.
The 24-year-old served time in jail for attempted sexual abuse of a minor and trespassing in 2010 and 2011, court records show.
He was charged with violating his probation this spring and was supposed to be on probation until 2014.
Police said the younger couple, who are in their twenties, went to a movie on Saturday night with their son and left the two-year-old in the care of Chea and Sreap.
They returned shortly before 8pm and found the door locked from the inside by the chain, but peeking through they could see the body of one of their grandparents.
The father told police he forced his way in through a window and discovered the bodies of Chea and Sreap.
They then discovered a naked man in the bedroom with their daughter.
His wife called 911 and police dispatchers heard screaming over the phone. She reported a man had broken into her home and killed her grandmother, Sreap.
The man was described as being naked with several tattoos.
The wife, who is pregnant, and her husband tried to keep the suspect from leaving and a struggle began, Police Sergeant Slawomir Markiewicz said.
The suspect, by then wearing boxer shorts, was able to get away after a few minutes of fighting, Markiewicz said.
Officers found Active about a block away.
'He did offer some resistance but he was arrested,' Markiewicz said.
The suspect apparently entered the apartment through a window, Markiewicz said.
The two-year-old was taken to hospital for a forensic physical examination, and it was discovered that she had also been physically assaulted.
She was taken to a hospital where she underwent surgery for her injuries.
Investigators were affected by the brutality and the ages of the victims, police department spokeswoman Anita Shell said.
'They said this was the worst thing they had ever seen in their lives, and these are seasoned detectives,' Shell said.
Police Sergeant Slawomir Markiewicz said yesterday that there are no indications that the victims were connected to the suspect.
'It doesn't appear that he knew them,' he said. 'It looks like a totally random act.'
The victims were part of an extended family that lived in a ground-floor, east Anchorage apartment with their granddaughter and her husband, who are the parents of the toddler and her four-year-old brother.
The younger couple's 90-year-old great-grandmother also lives in the apartment and was at home during the incident.
Markiewicz said the case is unusual.
'It's certainly very rare to see this kind of violence - a complete stranger, sexually assaulting and murdering someone,' he said.
The bodies of Sreap and Chea were taken to the state medical examiner's office for autopsies. Names of the toddler and her parents were withheld.