The cold case was reopened in October, and this week, aerial images and photographs showed a crew digging at the edge of a rural wooded field on the property of a vacant farm in Macomb Township.
One of the missing girls, Kimberly King, disappeared in 1979 when she was 12 years old.
But to other inmates, authorities say, Ream was a prison cell braggart who spoke frequently about the girls he'd killed and the shallow graves he buried them in - mysteries that have flummoxed investigators for almost four decades.
The FBI and other agencies are digging in a forest in Macomb Township for the remains of 12-year-old Kimberly King.
Cindy Zarzycki was last seen after being lured to a Dairy Queen in Eastpointe, just north of Detroit.
Three weeks before, Ream had been convicted of first-degree murder in Zarzycki's death but had not yet been sentenced.
The remains of the Zarzycki were found nearby in 2008 after Ream led police to the body.
Dwyer said Wednesday that the victims had no connections to each other.
"The suspect in this case also did brag about murdering four to six people to inmates where he is being housed", Dwyer said.
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That is where Ream buried the body of Cindy Zarzycki, a 13-year-old girl he killed in 1986.
Ream is now the primary suspect in King's case, in part because he was out on parole at the time of her disappearance.
"We have probable cause that this is a gravesite that Kimberly King and other young females were murdered and buried here", stated Commissioner Bill Dwyer.
The mystery of what happened to a 12 year-old girl who went missing nearly 40 years ago near Detroit in the USA could be about to be solved - along with the disappearance of up to six more girls. Dwyer repeated they're "cautiously optimistic" they will find remains. The FBI and other agencies spent Tuesday at the site about 30 miles (50 kilometers) northeast of downtown Detroit. He was free on parole on September 15, 1979, when King went missing.
Dwyer says four to six other girls could be buried there.
King's sister, Konnie Beyma, said she has been in contact with detectives.
King was reported missing while she was staying at her grandparents' home in Warren, Mich.
Warren police, the Federal Bureau of Investigation and other agencies spent Tuesday digging through dirt and vegetation at the site in Macomb Township for the remains of Kimberly King.