Police dashcam video unsealed Monday by a federal judge in Missouri shows a deeply disturbing encounter between a former Missouri cop who, during a routine traffic stop, tasered, dragged and callously dropped a teenage driver face first onto the pavement. The officer's actions during the arrest caused the teen to suffer brain damage.

Former Officer Timothy Runnels was sentenced last week to four years in prison over the September 2014 incident. The video above shows Runnels approaching a pulled-over vehicle being driven by Bryce Masters, then 17, who was on his way to play video games with a friend.

"I haven't done anything, officer," Masters says as Runnels attempts to forcibly remove the teen from his car. Masters is heard refusing multiple orders to exit his vehicle.

Runnels tells Masters he’s under arrest, but doesn't explain why. Eventually, Runnels says, “F**k it,” and discharges his stun gun on Masters, striking the teen near his heart.

As Masters slowly slumps out of the driver's seat, Runnels grabs the teen's cellphone -- which he was using to record the incident -- and flings it away. The teen then lies facedown on the ground, where Runnels eventually places him in handcuffs. According to court records, Runnels had his Taser deployed for about 20 seconds during the incident — the equivalent of four discharges of the nonlethal weapon.

Runnels then drags Masters' limp body to the side of the road, where he drops Masters face first onto the ground. With his hands cuffed behind his back and unable to break his fall, Masters lands on the pavement with a horrifying thud.

Police originally said that Runnels stopped the car because he believed the license plate was linked to an outstanding arrest warrant. Masters' parents argued that neither their son, nor their car, which was properly registered to the parents, had any warrants connected to them. Later Runnels said that he also smelled marijuana coming from the vehicle as part of his justification for placing Masters under arrest. He did find a small amount of pot on Masters when he searched his pockets.

Shortly after the incident took place, Independence Police Maj. Paul Thurman said the officer’s use of his stun gun was within department policy. But contradicting witness and police accounts of the incident surfaced and the FBI and U.S. Department of Justice launched investigations into the case. The DOJ then took the case before a federal grand jury last year and the panel indicted Runnels on counts of excessive force and obstruction of justice, over allegations that he'd made a false report and provided false statements to investigators.

There were no charges against Runnels for his stun gun use -- although it was the effects of the Taser that caused Masters to go into cardiac arrest, depriving his brain of oxygen for up to eight minutes and resulting in brain damage, The Intercept reported. Instead, feds argued that it was Runnels' deliberate drop of the teen while he was restrained and not posing a threat to others that violated his constitutional rights.

In September, Runnels pleaded guilty to violating Masters' rights, and last week, a federal judge sentenced him to four years in prison. Runnels, who remains free on bond, must surrender to federal prison by August.

“The defendant abused his authority as a law enforcement officer by depriving a minor of his constitutional rights and causing bodily harm,” said DOJ Civil Rights Division acting deputy Assistant Attorney General Vanita Gupta in a statement last week when Runnels was sentenced. “While the majority of law enforcement safeguards our communities with fidelity, the department will continue to vigorously investigate and prosecute officers who violate their oath by using excessive force.”

The dangers of electronic control weapons, like Tasers and other stun guns, have been widely studied by law enforcement oversight groups. In 2011, the Police Executive Research Forum, a nonprofit ­policy and research organization, partnered with the Justice Department to craft new guidelines on their use. Their report specifically cautions against discharging stun guns for extended periods of time.

“Officers must be trained to understand that repeated applications and continuous cycling of ECWs may increase the risk of death or serious injury and should be avoided,” the report reads.

During a hearing last month, Masters described how the brain damage he suffered continues to affect his sleep and memory.

“People tell me I’m different,” Masters said. “I feel different. I get in the car to go somewhere and then I’ll forget where I was going. I’ve missed job interviews because I forgot them.”

Runnels apologized to Masters' family during that same hearing, saying he was "deeply remorseful."

“At no point did I intend to hurt him," Runnels said. "But I did."