Police are remaining tight-lipped about the true nature of the operation to trawl a section of the River Tees.
A team of specialist divers have been working alongside a JCB digger while debris was hauled out of the river and sifted through at site at St Mark's Basin in Stockton.
Cleveland Police has confirmed the work is all part of a planned operation but refuse to confirm which investigation it is linked to.
The force spent the day working alongside colleagues from Northumbria Police’s Marine Unit and Cleveland Fire Brigade.
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Police tape was used to seal off the surrounding area while divers trawled through the murky waters in the river near to University Boulevard.
The operation came to a close for the day just before 4pm as darkness set in.
A Cleveland Police force spokesperson said: "Cleveland Police are working with partner organisations to conduct a planned search of the St Mark's Basin in Stockton as part on an ongoing investigation."
The force refused to confirm the nature of the investigation and were unable to say whether the specialist teams would return to carry out more digging and searching.
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