The detectives, who didn't know there was a videotape, denied slamming the man's head in to the ground in written police reports and under oath during court testimony.
"In law enforcement we say, 'You lie, you fly.' If your word is no good, there's no need for you as an officer," said Lou Reiter, a 20-year veteran of the Los Angeles Police Department and current police consultant. "They're going to face the consequences of possibly never being able to testify again in a criminal matter."
The video obtained by 9NEWS, shows Detectives Michael Cordova and James Costigan arresting John Heaney on April 4 after the officers say he ran a red light on his bicycle at 20th and Blake Streets.
The detectives admit they punched, kicked and choked Heaney during the arrest. In police reports, the officers say they used excessive force because Heaney punched Cordova in the nose. Heaney denies he hit the officers.
The video also shows Cordova pulling Heaney's hair, lifting up his head and slamming it into the ground when Heaney was lying on his stomach while officers held his hands held behind his back. Two of Heaney's teeth were broken off in the head slam.
"They had him in a position of disadvantage on the ground for handcuffing. So there would have been absolutely no justification for that use of force," said Reiter. "That use of force would have been vicious and vindictive."
Cordova claims Heaney broke his $169 Nike brand sunglasses in the scuffle. Heaney was charged with criminal mischief for the sunglasses and second degree assault for allegedly punching Cordova in the nose. The assault charge carries a three-year minimum prison sentence, according to attorney Lonn Heymann.
The Denver District Attorney's office dropped all the charges against Heaney on Friday after 9NEWS showed prosecutors and police the videotape of the arrest.
"Officers face difficult tasks every day and they have to be above petty animosities and anger," said 9NEWS legal analyst Scott Robinson. "If that's why this individual was injured, that's utterly inexcusable."
Use-of-force experts say that head slams should only be used in situations that would warrant the use of deadly force because they can seriously hurt people.
"You're dealing with some vulnerable areas, you're dealing with all of the orbital fractures that could occur, you've got nose fractures, you've got all the mouth and teeth injuries and as a consequence, you're doing to induce serious injury to that person," said Reiter. "You're going to have to have justification to use that force."
Cordova joined the Denver Police Department eight years ago. In May 2005, the department awarded him the medal of honor for "displaying an act of courage that clearly distinguishes gallantry beyond the call of duty." Cordova and another officer pulled people out of a burning house.
Costigan became a police officer in 2000. Costigan and Cordova are on the vice-narcotics squad and were working on an undercover scalping sting when they arrested Heaney. The police department said it can not determine how many use of force reports have been filed against the vice-narcotics squad.
The city's independent monitor will monitor the police internal investigation and the results will be reviewed by Denver Manager of Safety Al LaCabe, according to Mayor John Hickenlooper's office.
"We have asked all parties to act as quickly as possible so that we may fully understand the facts of this situation and move forward as those facts warrant," said Hickenlooper.
Denver's use of force policy says nothing about punching, kicking, pulling hair or head slams. The decision is left up to each officer's discretion depending on the circumstance, according to the Denver Office of the Independent Monitor.
"The use of force policy by design and necessity is fairly general. It has to be because it's impossible to identify all of the circumstances that would happen on the street," said Richard Rosenthal, independent monitor. "It's a case-by-case analysis."
The Police Assessment Resource Center (PARK) recommended the Denver Police Department broaden its use of force policy to include the constitutional right of people to be free from all forms of excessive force.
In a report published in June, PARK recommended the city add this phrase to its policy: "An officer shall use only that degree of force necessary and reasonable under the circumstances."
PARK also said that Denver's Internal Affairs investigations should include an analysis of its policy and examine practices to lessen the risk of injury to officers or suspects. Overall, the report noted that the Denver Police Department meets and even exceeds national standards in many areas.
The officers chose not to comment for this story.