According to TMZ, he was killed after his jeep went over the side of Pacific Coast Highway Monday at 4:33 p.m.
Ryan, who reportedly was 50, was trapped in the vehicle and suffered major head injuries, TMZ reports. "The Ventura County Fire Department tried hoisting Ryan using a helicopter, but the attempt was called off," says the site. "Ryan was pronounced dead at the scene."
Minutes before the crash, he Twittered, "After 25 years of driving by, I finally hiked to the top of the giant sand dune on the pch west of Malibu. Much harder than it looks! Whew!"
The surgeon - who performed on celebs including Gene Simmons, Shauna Sand and Vince Neil - came under fire this year for altering Montag's appearance so drastically.
"I am devastated to hear the news of Dr. Frank Ryan's death," Montag, 23, wrote on Twitter. "He was the most amazing person I have ever known. He was an angel and changed my life and the lives of everyone he met. He was the most brilliant talented surgeon who will ever exist. Dr. Frank Ryan changed the world. My thoughts and prayers go out to his mother, family, friends, and anyone who was ever blessed enough to meet him. He is in a better place."
Last fall, The Hills star underwent nose, cheekbone and chin jobs, as well as an eyebrow lift, breast enlargement and fat injections, among other procedures.
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In March, she told Jay Leno she wanted bigger breasts, but "I legally can't right now. The limit is 800cc and I have 700cc." As for other surgeries, she said, "I think I've reached the point [where I can stop]. I think I'm good for awhile. I'm surger-ied out."
A month later, E!'s Dr. 90210 (real name: Robert Rey) told UsMagazine.com he wouldn't have performed more than three surgeries on the reality star.
"What worries me is the commentary she is making to young girls," he told Us.
Dr. Rey didn't slam her surgeon. "The issue of too many procedures in the operating room have [been debated] and forth," he said, "but he is a great surgeon -- that's not the issue."
Montag filed for divorce from husband Spencer Pratt last month, citing "irreconcilable differences."
You can read Dr. Ryan's tweets minutes before the crash @DrFrankRyan.
Heidi has updated her Twitter: "I am devastated to hear the news of Dr. Frank Ryan's death. He was the most amazing person I have ever known. He was an angel and changed my life and the lives of everyone he met. He was the most brilliant talented surgeon who will ever exist. Dr. Frank Ryan changed the world."