Spierer, 21, went missing in Bloomington during the early morning hours of June 3, 2011. She remains missing.
The skull, which was found by a fisherman, has not yet been identified but has been sent to a forensic anthropologist, the Star reported.
Although results could take six to eight weeks, forensic anthropologists will help the Marion County coroner identify race, age and gender.
No additional remains were found in the river.
Several other incidents have caused authorities to question whether they found Spierer or a connection to the case. In May, officers from the Bloomington Police Department were in contact with authorities in Lafayette, La., after University of Louisiana-Lafayette student Michaela “Mickey” Shunick, 21, was reported missing.
Also in May, police investigated a possible connection between Spierer’s disappearance and Clyde Gibson of New Albany, Ind., who is accused of killing three women.
In April, remains found in Newton, Ill., were discovered, also piquing the interest of Bloomington authorities and the Spierer family.
But so far, none of the potential leads have brought a resolution.
“When will you end this nightmare for our family? We will never give up,” a Facebook post by the Spierer family said. “We are just as determined as we were on June 3, 2011. You need to know that. It’s our promise to you. For now we continue to wait. The Spierers.”
— Mark Keierleber
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