KIRKWOOD, Calif. (AP) — A mother and her 7-year-old son died when a massive block of snow fell from a roof and buried them just feet from the front door of their Northern California mountain condo, authorities said.

Olga Perkovic, 50, and her son, Aaron Goodstein, had been skiing in the Sierra Nevada near the Nevada state line and were returning to the condo Sunday when a chunk about the size of a trailer fell from the roof, burying them under about 3 feet (0.91 meters) of snow, the Alpine County Sheriff's Department said.

"It was a freak accident," Undersheriff Spencer Pace said Monday.

He said warming temperatures often cause snow to slide off roofs, occasionally injuring people. But he said neither he nor the sheriff can recall sliding snow ever killing anyone in the three decades they have been there.

Perkovic's mother, who was staying in the condo with the family, reported the pair missing, Pace said. Rescuers searched the nearby Kirkwood Ski Resort for hours because the mother and child's last known location was a ski lift where they had scanned their tickets around 4 p.m.

Pace said it appears the two skied an "alternate" route home from the slopes that took them between buildings.

Around 9 p.m., a neighbor spotted ski gloves next to the condo, realized they were beneath the snow and called 911. The mother and son from San Francisco were airlifted to a hospital, where they were declared dead, Pace said.

Perkovic, who had a doctorate in physics from Cornell University, had last worked as a senior adviser at Zymergen, a startup that focuses on molecular technology.

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"Olga was a valuable member of our community at Zymergen, a trusted adviser, and most importantly, an old and close friend of mine and so many others," said Josh Hoffman, Zymergen CEO. "She will be missed by all of us. Our thoughts are with her husband David and her daughters, Sophie and Daria."

Her son, Aaron, was a first-grade student at French American International School, spokeswoman Keelee Wrenn confirmed Tuesday. She said the school has grief counselors on campus.

The mother and son were the third and fourth skiers to die at California resorts since a major snowstorm late last week.

The region's largest storm of the winter dumped more than 6 feet (2 meters) of snow in the area over the last week, according to the Kirkwood Ski Resort, which is about 180 miles (290 kilometers) east of San Francisco.

Avalanches also briefly closed Squaw Valley and Mammoth Mountain in recent days.

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