Inquiries are focusing on whether the 38ft-long MFV Quest was sabotaged after it was left moored by its owner at Acarsaid pier on Eriskay on March 1.
The Oban-registered vessel had been fishing in the Minch for the previous six weeks. Nobody was hurt in the sinking and she has since been floated.
Detectives were sent to the island, made famous by the film Whisky Galore, where the SS Politician famously sank off Eriskay in 1941.
The SS Politician was headed for Jamaica with 28,000 cases of whisky when it ran aground on the northern side of Eriskay in bad weather.
A Police Scotland spokesman yesterday confirmed the incident is now being treated as suspicious. He said: "Police can confirm that following the recovery and examination of the MFV Quest that they are treating the sinking as suspicious and that an investigation into the matter is ongoing.
"Police are keen to speak to anyone who has any information about the incident."
A 23-feet long Eriskay creel boat, the Reul - which was moored alongside Quest was dragged down with Quest.
The smaller fibreglass vessel was refloated and was towed to Mallaig for repairs.
Mark Woods, owner of Quest, has previously spoken of his shock at the sinkings.
He said at the time he was "absolutely gutted" by the sinking as there was no indication of any problems with the vessel.
He had said she was in good condition and the weather was fine at the time.
Airbags were used to lift her off the seabed.
Mr Woods added: "I just hope that whoever is responsible for this is brought to justice."
Scottish author Compton Mackenzie, published the novel Whisky Galore in 1947, which was loosely based on the SS Politician wreck. It was adapted for the cinema in 1949.