An exhaustive postmortem on the humpback whale calf euthanized after its stranding on the shore of East Hampton last spring has found no conclusive health reason why it beached itself.

Despite earlier suggestions the whale may have stranded itself because of sickness or injury, tests carried out by a federal agency found the whale had been in "good body condition" although its stomach was largely empty.

Unable to move or rehabilitate it, officials used several toxic doses of drugs and three blasts from a .577 rifle to euthanize the 13-ton whale. A toxic dart lost in the surf was never found.

"This is still a bit of a mystery," said Trevor Spradlin, a spokesman for the National Oceanic and Atmospheric Administration's National Marine Fisheries Service, which stands by the decision to kill the whale. "The reality is it was suffering on the beach. It was slowly dying. There was no alternative."

The report, released to Newsday after a Freedom of Information Act request, said "the immediate cause of death of this stranded cetacean was euthanasia resulting in cardiopulmonary collapse. . . . A cause of stranding was not determined in this case."

For a windy, stormy half-week in early April, scientists scrambled as the whale thrashed and rolled in heavy surf. Despite local protests, officials ruled out helping the whale back to sea, concluding whales often strand themselves because of sickness or injury. The report rules out both.

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"There was no evidence of infectious or inflammatory disease or biotoxicosis," it states.

The report determined the whale to be younger than previously believed - it was from 1 to 2 years old, rather than the 2 to 5 years officials indicated in April.

The report reiterates earlier claims that the location and circumstances of the stranding made a rescue difficult.

The necropsy found the whale's right ear was fractured, likely the result of the rifle shots. It found fluid "but little solid material" in the intestine. Round worms were found around the kidneys.

Fisheries officials, in trying to determine whether a boat strike or other accident injured the whale, went so far as to ask the U.S. Navy if it had conducted any military exercises in the area.

The Navy "reported no active sonar within 100 [nautical miles] and previous 72 hours," the report states.