St. Petersburg, FL - You've probably seen one while out on a Saturday night: a DUI checkpoint.

Law enforcement uses these checkpoints to get drunk drivers off the road, but the 10 News Investigators have found one checkpoint that did just the opposite. How? It was an illegal checkpoint, and if that wasn't bad enough, officers for the Florida Highway Patrol, Tarpon Springs Police Department, and the Pasco County Sheriff's Office are accused of swearing under oath they did nothing wrong.

The idea behind a DUI checkpoint is that it allows officers to randomly pull cars over and check for drunk drivers. But DUI attorneys and experts tell 10 News a DUI checkpoint operated last December in Pasco County was flat out illegal. "No one is going to say that DUIs are not a problem, that they're not serious, that they're not important," said Tampa DUI attorney Jason Sammis. "But the good guys always have to play by the rules. The good guys have to balance their need to stop DUIs with the personal rights and freedoms of people."

In 1990, the Supreme Court ruled in a 6-to-3 decision that sobriety checkpoints are constitutional, but law enforcement had to follow certain rules. One of the rules is law enforcement must file an operational plan before they launch the checkpoint that describes how they will randomly select drivers for field tests.

Experts say filing a plan beforehand insures that police can't profile drivers based on race or any other factor, such as a bumper sticker they don't like. "It's got to be a random stop. They have to pick every third car, every fourth car, and if they deviate from that plan it is basically an illegal checkpoint by the U.S. Supreme Court," said DUI expert Stephan Daniels.

So let's go back to that DUI checkpoint last December. Pasco Sheriff's deputies, Florida Highway Patrol troopers, and Tarpon Springs Police officers were in charge. The operational plan they filed stated they were going to pull over every third vehicle. Daniels noticed on the police's own squad car video recordings that the three agencies were breaking the rules. The video clearly shows the flagman at the front of the checkpoint wasn't following the plan and flagging in three and four cars at the time.

But once the state attorney's office was notified that the operational plan wasn't followed and they were pulling over more than every third car, instead of dropping the cases, the state attorney's office took a new tactic and it got ten law enforcement officers who worked that night to sign affidavits saying the operational plan was followed, which was a bald-faced lie.

Attorney Jason Sammis said he found it "disturbing" when we asked him what it does to our system when someone is so zealous about trying to get a conviction they would lie on an affidavit and get law enforcement officers to swear what is true is not true.

Sammis, who was representing one of the drivers charged with DUI, wouldn't let it go. He insisted the State Attorney's Office take another look at the video, and prosecutors admitted the checkpoint was illegal.

A spokesman says Assistant State Attorney Vin Petty missed that fact while he was at the checkpoint, and when he first watched the video, and that's why he had officers sign the affidavits. The police agencies also claim they had no idea they were violating the law.

"There were law enforcement officers at the scene, and they were there the entire night, and they saw no violations of this every third car rule," said Sammis, whose client has had his case dropped. It also jeopardized the convictions of everyone arrested during that checkpoint.

"Potentially they could put impaired drivers back on the road. The very thing they are trying to stop they are participating in now," said DUI expert Daniels.