The mass die-off of nearly 900 dolphins and porpoises along Peru's coast remains unsolved, Peru's government marine research agency says.
Tuesday's final report by the Sea Institute ruled out viral and bacterial infections, human intervention, pesticides or heavy metals as causes for the deaths, which were first noticed on Feb. 7 and continued through mid-April.
The report speculated that biotoxins, algae blooms or an unknown emerging disease could be to blame.
The Peruvian environmental group Orca, which first alerted the public to the deaths, insists that seismic testing used in oil exploration was likely the cause.
But the institute said that experts found no evidence that any of the deaths were a result of seismic soundings, which involve shooting compressed air at the sea floor.
There were no signs of internal hemorrhages or brain lesions that would be compatible with damage from such tests.
But the researchers said they did notice damage to some plankton where the soundings were done.
Environmental group blames seismic testing
Orca contested those findings in its own report on Tuesday, saying it had independently confirmed hemorrhages and middle-ear infections as well as the presence of air bubbles in internal organs and severe lung damage.
Several leading Peruvian scientists complained that the government agency was late in gathering samples, making it harder to determine the cause of death because the tissue tested was so badly decomposed.
The Sea Institute based its findings on autopsies of just two dead dolphins, which were collected in mid-April, while Orca said it gathered the first of the samples it tested on Feb. 12.
Seismic testing in the area was conducted between Feb. 7 and April 8 by Houston-based BPZ Energy.
The institute report said the testing occurred 80 to 130 kilometres off shore and that the equipment used was calibrated in those waters between Jan. 31 and Feb. 7.
It said testing also ruled out morbillivirus, a type of distemper that some government officials had suggested as a likely cause long before kits arrived from the United States to check for it.