Nearly 900 dolphins that washed up along Peru\'s northern coast since the start of the year died of natural causes, a top official said Tuesday, citing a government report. Environmental groups, however, remained unconvinced and said they were certain the massive dolphin die-off was linked to offshore oil exploration in the area. \"We have reached the conclusion that the deaths were from natural causes,\" said Gladys Trevino, Peru\'s Production Minister, speaking on local radio as she announced the results of a government investigation. Trevino said the study by the government-run Institute of Peru\'s Ocean (IMARPE) had ruled out other explanations such as offshore oil exploration or viral or bacterial infections. \"It\'s not the first time that this has happened,\" she said, pointing to what she said were similar mass dolphin die-offs in Australia, New Zealand and other countries. Peruvian officials had already suggested that the dolphins, along with some 5,000 dead sea birds -- mostly pelicans -- died due to the effects of rising temperatures in Pacific waters. The Scientific Organization for the Conservation of Aquatic Animals, known by its Spanish acronym ORCA, has said the dolphin deaths were linked to the noise from oil exploration. A representative from ORCA said the group earlier this month tested 30 dead dolphins and found broken ears and damaged organs consistent with the victims suffering \"the bends,\" also known as decompression sickness.