(ABC News) -- For a third time in a little over a year, body parts have been found in the sewers surrounding Detroit.
On Wednesday, workers found human remains while working in the sewer system in Sterling Heights, Mich.
This is the second time body parts have been discovered in the Sterling Heights sewer system. Human remains were also found in the nearby city of Warren, Mich., in late 2012.
None of the bodies have been identified.
Lt. Kevin Reese of the Sterling Heights Police Department told ABC NEWS affiliate WXYZ-TV in Detroit that finding the source could be difficult due to the sewer’s size and the fact that the remains were very small pieces.
“What is the most frustrating part for myself and my investigators is that we don’t know where they’re coming from,” Reese told WXYZ-TV. “These pieces could be possibly flushed down the toilet.”
Police Work to ID Body Parts found in Michigan Sewer
The sewer system runs between multiple towns.
In late 2012 workers found at least a dozen human remains, some with tattoos, that were never identified.
The grisly findings have made some residents in the area nervous.
“There’s a lot of crazy people in the world,” resident Robbie Cipriano told WXYZ-TV.
However, because there have been no reports of missing people, and due to the size of the remains, police now suspect that they may not be from murder victims but instead might be medical waste.
“There is no vital organs or appendages. No one has come forward at that there are missing people,” Reese said. “We’re starting to lean a little bit towards it could possibly be medical waste being disposed of illegally.”