A sailboat competing in the annual Newport to Ensenada Yacht Race likely ran aground before it splintered and sank, according to an official report released Tuesday.

On April 28, the Aegean crashed into the rocky shore off North Coronado Island, one of several Mexican-controlled islands about 15 miles south of San Diego Bay, according to an independent panel investigation by U.S. Sailing, the sport's governing body.

All four aboard the 37-foot, Redondo Beach-based vessel were killed: skipper Theo Mavromatis, 49, of Redondo Beach; William Reed Johnson, 57, of Torrance; Joseph Lester Stewart, 64, of Bradenton, Fla.; and Kevin Rudolph, 53, of Manhattan Beach.

Mavromatis, Johnson and Rudolph died from blunt-force trauma, and Stewart drowned, according to San Diego County's medical examiner.

In the days following the Aegean's accident, race organizers surmised that the sailboat was struck by a large commercial ship because its remnants were decimated so completely. Small splinters were found floating in the ocean.

The first to point out that the Aegean possibly ran aground was fellow Ensenada racer Scot Tempesta, who formed his opinion after monitoring the Aegean's GPS path that was posted on a commercial tracking website. Tempesta then published his theory on his website, Sailing Anarchy.

The panel's five members also reviewed the Aegean's GPS device to understand the vessel's course that night, said U.S. Sailing spokesman Jake Fish. Race organizers and Coast Guard investigators in San Diego were also consulted.

"There is no evidence that supports any other conclusion," Fish said.

A full report on the accident is expected by the end of July.

The Coast Guard's investigation into the accident remains open.

The wreck was the first fatal accident in the race's 65-year history.

lauren.williams@latimes.com

Twitter: @lawilliams30