The snubfin dolphins were found tied to mangroves and weighed down by concrete blocks at Toolakea Beach, north of Townsville, last June.
At the time, the Great Barrier Reef Marine Park Authority said it appeared the dolphins had accidentally been caught in fishing nets and those responsible had tried to conceal the bodies.
Environmental group WWF had offered a $5000 reward for information leading to the arrest of the people involved.
But in a statement to AAP on Thursday, the authority said its investigation had been closed without a resolution.
“The investigation was unable to find an eyewitness or sufficient evidence to identify anyone responsible for this offence,” the statement said.
“The case has now been closed.
“(However) the case will be reopened if new information is able to identify the responsible person or persons.”