Five of the junior officers are in a critical condition, with the most serious injuries involving loss of limbs, broken bones and head trauma.
Four others have injuries described as “moderate”, while a further 16 suffered minor injuries.
The shocking scenes unfolded in Southern California on Wednesday at around 6.30am when a group of 75 law enforcement recruits were on their morning exercise.
Images show a written off vehicle badly damaged at the bonnet and front bumper, while firefighters, medics and detectives investigate the road.
The driver, who has not been identified, is in police custody.
“We know for sure we have a 22-year-old male driver that was going the wrong way and ran into the class,” Sheila Kelliher, a spokesperson for the Los Angeles fire department, said on Wednesday morning.
Aerial footage of the scene in Whittier showed running shoes and backpacks strewn on the ground.
Sheriff Alex Villanueva said the Honda CRV involved looked “like an airplane wreck” after the incident, during which it also crashed into a pole.
Sheriff Villanueva said: “It is hard to see because these young people are getting ready to go put themselves in the line of danger in their career, and who knows that while you’re training to do that, you’ve put yourself in harm’s way.
“So my heart goes out to all of them as they pursue this career. I hope they all have speedy recoveries and that they’re in good shape.”
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All trainee sheriffs, who were in their eighth week of training, had reflective vests on when they were hit.
One of the injured is on a ventilator in hospital.
The Los Angeles Police Department offered its support to the trainees affected by the crash.
In a statement, it said: “Our thoughts are with those impacted by this morning’s events involving the Sheriff’s Recruits.
“As we await more information on their conditions, we are wishing everyone a quick recovery.”