Despite what she called an “unusual mortality event” killing dolphins in the Gulf of Mexico, the top federal scientist investigating the deaths, revealed Wednesday that the government has yet to send any tissue samples for laboratory testing to determine a cause. The National Oceanic and Atmospheric Administration’s Blair Mase blamed the delay on complications related to oil spill litigation.

A letter sent by NOAA to groups authorized to collect tissue samples from dead dolphins described the work as “a criminal investigation,” according to Mase.

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