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It was one of the biggest mass whale deaths in Irish history, but four days after the bodies of 33 whales washed ashore on Rutland Island, off County Donegal on the northwest coast of Ireland, environmentalists are still stumped over what could have killed them. As experts run tests on the group of deep-diving pilot whales, believed to have been dead before they reached the shore early Saturday, theories on the culprit range from illness to a deadly storm. But some vocal critics are pointing the finger at the Royal Navy, and saying that its ships' sonar equipment is to blame.

The first thing experts are trying to establish is whether or not the Rutland Island whales are the same group that was spotted about to strand off of Scotland's South Uist coast a week ago. The Scotland whales were being monitored by British Divers Marine Life Rescue, but after a storm hit they went off the radar. If the pod of whales that washed up on Rutland Island is the same as that in distress off the Scottish coast, it could provide a vital clue about what killed them. "Pilot whales are an offshore species and for them to be seen so close to shore indicates that there was something seriously wrong," says Aoife Foley, a whale expert with the Galway Mayo Institute of Technology who is involved in testing blubber and teeth samples from the Rutland Island whales.

The beaching of whales is not an unprecedented event along the Irish coastline — 40 whales died when they got stranded in County Kerry in 2001. But environmentalists are concerned that Royal Navy sonar equipment may have played a role in this latest mass death. "There is a naval testing ground near South Uist, where the original pod were spotted," says Simon Berrow, coordinator for the Irish Whale and Dolphin Group (IWDG). "Therefore, if this is the same group, their death is likely to be an affect of acoustic trauma." Low-frequency sonar waves can emit noise as loud as a jumbo jet taking off, which can severely damage whales' inner ears. Whales use their hearing to follow migratory routes, care for their young and find food sources and other whales over great distances. If their hearing is damaged, it becomes impossible for them to live.

In the past, research has proven that sonar waves can interfere with whales and dolphins, causing them to beach. Low-frequency sonar has been implicated in many mass-stranding events all around the world, including those in the U.S., the Canary Islands and Australia. In 2007, after five beach whales were found dead along California's coast, a court ordered a two-year ban on sonar testing by the U.S. Navy during training.

Scientists and researchers examining the samples and photographic evidence of the Rutland Island whales, along with the data on those monitored off the coast of Scotland, hope to show that the Royal Navy's sonar use drove the whales from Scotland's South Uist to Donegal, where, injured and disoriented, they got stranded on the shore and died. "Something affected these whales," says expert Foley. But a spokesman for the Royal Navy tells TIME in a statement that while there was a ship off the Scottish coast at the Clyde naval base in Faslane, "no sonar was being used" on the days surrounding the mass stranding.

And Darlene Ketten, senior scientist at the Massachusetts-based Woods Hole Oceanographic Institution, notes that sonar seems to affect only beach whales, not pilot whales like those found dead in Donegal. "A sonar exercise involves ships moving along the coast, back and forth almost in a herding maneuver," she says. "Beach whales seem particularly sensitive to this and they end up onshore." Ketten, who has previously worked with the IWDG and has spoken out about the U.S. Navy's controversial use of sonar in the Caribbean, believes that the whales on Rutland Island may simply have experienced a "classic stranding." "One vital tendency of pilot whales is group cohesion," she says. "If one animal goes onshore, they often pull other animals in with them."

Another clue that points to a culprit other than sonar is the fact that, as Ketten notes, the Rutland Island whales were discovered buried under sand: "This could suggest that they came onshore during a storm and got stuck, resulting in an incredibly tragic event." But until environmentalists and experts can figure out the cause, there could be more tragedy to come on the Irish coast.

• Claire Mc Cormack
• After 33 dead whales were discovered washed up on the coast of Rutland Island on Saturday, environmentalists and experts have been searching for clues on what killed them. But many already think they know the culprit: The Royal Navy