A human skull was discovered by a crew while digging at the site, East Chicago police said.
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Officers responded to the refinery, located at 2815 Indianapolis Blvd in Whiting, Indiana, around 9:45 p.m.
Maintenance was being performed on the pipe system when the skull was found encased in the gravel near a refuse bin, police said.
The skull was given to the Indiana Lake County Prosecutor's Homicide Task Force and Lake County Coroner's Office, police said.
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No further information was immediately available.
SEE ALSO | Indiana environmental department holds meeting to hear public concerns over Whiting BP oil refinery
Authorities continue to investigate.
ABC7 has reached out to BP for comment.