Experts remained puzzled Tuesday by the death of an endangered fin whale that washed ashore near Stinson Beach in Marin County with signs of blunt force injury on the underside of its body.

The juvenile whale had a large swath of bruising under its skin, from its jaw down along the right side of its ribs, over an area about 8 feet long and 4 or 5 feet wide, said Shawn Johnson, director of veterinary science for the Sausalito-based Marine Mammal Center.

But a team of veterinarians who examined it did not come across any broken bones that would be typical had it been struck by an ocean-going vessel — perhaps the most likely cause of the whale's death.

Other possibilities might include aggressive play or maybe a killer whale, the fin whale's only predator.

"We're trying to investigate with some whale biologists if they can give us some other ideas of what it might be besides ship strike," Johnson said.

The young male whale likely came ashore on county-owned Upton Beach around midnight Sunday, with the high tide, experts said.

It was reported to officials around 7 a.m. Monday by a resident of the Seadrift residential community on the north end of Stinson Beach, Marine Mammal Center spokeswoman Kate Harle said.

The massive creature was still alive, drawing a large, curious crowd as it occasionally thrashed in the surf, but it expired within hours.

Veterinarians conducting a post-mortem necropsy on the 20-ton whale found no external signs of trauma or abnormality that would explain the whale's stranding, and in fact found the animal "in great body condition," Johnson said. It had recently eaten, and "had lots of fat — internal fat, and blubber," he said.

The county had sent over heavy equipment for use in burying the carcass, which made it possible to turn the animal over, permitting the scientists a rare opportunity to inspect its underside.

Once they started removing its skin, they could see traumatized tissue around the sternum, hemorrhaging around the heart and air bubbles in the subcutaneous, connective tissue indicative of trauma.

Fin whales are an endangered species found in oceans around the world. They typically dwell in deep waters well off-shore and can live up to 90 years.

The Marine Mammal Center has responded to four fin whales that have died around the greater Bay Area since 2010<NO1>, including one on Ocean Beach in San Francisco and another in Oakland in 2010, and one at Palomar Beach in the Point Reyes National Seashore last year<NO>.

Two of them died from ship strikes, Harle said. The cause of death in a third case was never determined.

County parks Director Linda Dahl said many people may question the decision to bury the whale on the beach, but said the only other alternative would be to tow it to sea, with a high likelihood it would only wash ashore again once it was more difficult to manage because of decomposition.

You can reach Staff Writer Mary Callahan at 521-5249 or