Three gun safes have been found abandoned in the Collie River near Griffin Bridge.
Free diver Mark O'Hara stumbled across the safes last week while on an early morning dive cleaning up the Collie River bed.
Collie police acting senior sergeant Terry Townsend said local officers were still investigating the discovery at the time of print.
"Two of the safes appear to have been there for a considerable amount of time by the look of the rust build up, but one looks to be discarded relatively recently," he said.
"Officers are making enquiries on the issues because there have been a couple of burglaries and thefts of gun safes across the district recently."
Acting snr sgnt Townsend said the investigation would prove difficult due to a lack of evidence from being discarded in the water.
"Finding the safes close to town indicates that the persons responsible for the theft is probably either a local to the region or frequents the region quite regularly.
"Thankfully we don't get many guns stolen around here which is a good thing, but it is a concern for us that whoever put them there is close by."