A man suspected of killing his two children and two step-children on Wednesday at the family's home in the Detroit suburb of Dearborn Heights called authorities to report the slayings, police said.
He also attacked the children's mother but she survived with non-life threatening injuries, Captain Michael Petri of the city's police department told a news conference.
The suspect, whose name and age were withheld by authorities, called police at about 1:30 a.m. local time on Wednesday to say he had shot members of his family, Petri said.
According to local media, the man was sitting on the front porch of the house when officers arrived. Police said he surrendered without incident.
When officers entered the house, they found the dead children and their mother. They tried in vain to revive at least two of the children, the Detroit Free Press reported.
The newspaper reported that the woman had filed for divorce in August.
Officials from Dearborn Heights, a community of 56,000, said they have offered psychological counseling to the officers and firefighters who found the bodies.
"This is a very rare occurrence and it's just difficult to understand the motivation ... I don’t understand what happened in this household," Dearborn Heights Mayor Dan Paletko told reporters.
Petri said the killings might have stemmed from a domestic violence incident, but he declined to elaborate.
Detroit television station WXYZ and other local media reported that police initially said the man's two teenage stepchildren, a 17-year-old girl and a 19-year-old man, were shot dead, and his two biological daughters, ages 4 and 6, died of carbon monoxide poisoning.
The Detroit Free Press reported that police initially said the teens were discovered bound in the basement, where their mother was found with knife and gunshot wounds.
At the news conference Petri declined to confirm the manner of death of the children or the nature of the woman's injuries, although he said she was likely to physically recover. He did not release the victims' names.
The suspect is expected to be arraigned in court later this week.
(Reporting by Alex Dobuzinskis in Los Angeles and Brendan O'Brien in Milwaukee; Editing by Louise Ireland, Toni Reinhold)