Scientists and Peruvian officials are investigating a mass die-off of hundreds of dolphins along the South American country's coast.
A total of 877 dolphin carcasses have been counted recently along the shore in the northern regions of Piura and Lambayeque, Deputy Environment Minister Gabriel Quijandria said.
Officials have been studying possible factors in the dolphins' deaths including a virus or seismic oil exploration that has recently been carried out off northern Peru.
An analysis of the beached dolphins' internal organs hasn't found the sort of symptoms that experts have seen in other cases when dolphins have been affected by seismic tests, Mr Quijandria said in a radio interview.
He said experts are studying whether the animals could have succumbed to a virus.
"So far, it's the most probable hypothesis, and it isn't the first time it's happened. There have been cases in Peru, in Mexico, the United States," Mr Quijandria said.
He said the seismic tests produce underwater noise that can harm dolphins. But he also said that in Peru it is the first time such dolphin deaths have coincided with seismic work and that the dolphins began dying before the tests started.
Carlos Yaipen, who leads the non-governmental organisation Orca, said the beached dolphins began appearing in January.
Dolphins have had broken bones in their ears and some of their organs have been collapsed, suggesting that shock waves generated by the seismic tests could have killed them, Mr Yaipen said.
However, Patricia Majluf, the government's deputy fisheries minister, said that based on the available evidence officials have not been able to pin down any relationship to oil exploration.
Government officials have said scientists are carrying out further studies that they expect to finish in about two weeks.