Washington state investigators are examining nets heavily covered with mussels and other sea life as a cause in the collapse of Cooke Aquaculture's net pens last summer that released thousands of salmon into the sea, according to The Seattle Times.

Photographs obtained by the newspaper under a public-records request show portions of the nets at Cooke Aquaculture’s fish farm in Skagit County so fouled with kelp, algae and mussels that the net was no longer visible, said the newspaper.

Cooke is required, under the terms of its lease with the state, to maintain its farms in a clean and safe condition.