Arial footage showed that several riders hit a parked car after taking a left turn at high speed. The car was parked close to the corner, in the area riders cross to the right of the road as they exit the corner. They had little time to see and avoid the vehicle.
Riders from the Katusha, BMC and Orica-Scott teams were involved with other riders blocked or slowed by the incident.
The BMC team confirmed that Bookwalter and Diller hit the car, with Bookwalter being taken to hospital. He was sitting after the crash and did not appear seriously injured.
"It looked like Brent hit the edge of the car coming out of the corner and then flew forward and was lying near the passenger side door," said BMC team doctor Scott Major. "He didn't go into the windshield, but he did hit his head and has a mild-to-moderate concussion, which is the biggest concern and we will continue to monitor him now he is out of the hospital."
In a statement released by the team, Bookwalter said he doesn't entirely remember the crash.
"I remember, at the last moment, hitting the car and then I was on the ground," Bookwalter said. "I instinctively tried to get back up but my head was pounding, and I was feeling a bit fuzzy."
Major said Bookwalter has some small lacerations, including one on his shoulder that required stitches, as well as some road rash and a deep thigh contusion. Bookwalter underwent X-rays at the hospital, and everything came back negative, so there are no fractures and all his injuries appear superficial.
"In terms of his recovery, I think it will be good for him to be back on the bike in the next four to six days to spin the legs, but he will definitely need a couple of days rest," Major said.
Bookwalter said he is very sore but is slowly "getting back to myself."
"As disappointed as I am to crash out of the race, I can be thankful that I had a big round of X-rays and there are no fractures, and I won't have a prolonged period of recovery," Bookwalter said. "I am definitely in some pain but thankful that it wasn't as bad as it maybe could have been."