Gulfport, Mississippi - The number of dead dolphins washing up along the Gulf Coast continues to climb.

As of Tuesday, 70 dolphins, including 52 calves, have died in Mississippi and Alabama waters. The cause of their deaths remains a mystery, and samples taken from dead animals still have not been tested.

WLOX News has now obtained a letter from NOAA, which we first told you about last week. The letter was sent to marine institutes and orders researchers to hold the samples they've taken until federal officials decide their next step.

The letter cites an active criminal investigation by the Justice Department against BP and others involved in the spill. It also details a civil lawsuit the department has filed, which seeks civil penalties under the Clean Water Act and the Oil Pollution Act.

Click here(PDF) to read the original letter from NOAA.