MIAMI -- Miami-Dade County demanded millions of dollars from Wackenhut Security on Thursday, accusing the South Florida company of over-billing taxpayers for years.

Related Content: Read The Audit

NBC6 Consumer Investigative Unit reporter Jeff Burnside reported an NBC6 investigation on Wackenhut's over-billing last year.

In the latest development in the case, a massive county audit confirmed the previous allegations that Wackenhut conspired to commit fraud on Miami-Dade County taxpayers.

The audit's findings could cost the company its lucrative contract for Metrorail and others, totaling tens of millions of dollars.

Security guards and managers with the company said the emotional cost is impossible to quantify. Some are suing Wackenhut because, they said, they were fired after speaking up about what they felt was improper over-billing.

The Metrorail guards said what is quantifiable are the hours that they actually worked versus what Wackenhut Corp. billed Miami-Dade County taxpayers for.

The county audit, years in the making, reviewed a 2 percent sample of Wackenhut's invoices and concluded that $6.26 million was stolen from county taxpayers over three years of a seven year Metrorail contract.

"We're already moving in the direction right now of canceling all Wackenhut contracts with the county and going after our money," Miami-Dade County Mayor Carlos Alvarez said.

The audit found Wackenhut systematically used guards to cover parts of shifts, and then billed the county for the entire shift, or billed for two guards when there was only one.

Alvarez said it left passengers unprotected at times.

"It's a very serious allegation, not only because of the cost factor, but because of the security factor," he said.

A law firm that filed a whistleblower lawsuit against Wackenhut reviewed all of the company's invoices over five years. Attorney Mark Vieth said he believes taxpayers have had more than $17 million taken from them.

"It would seem to me that a fraud of this scale involving this amount of people and this amount of money could not have occurred but for the cooperation of county employees," Vieth said.

Alvarez confirmed that criminal investigations are under way that may include county employees and Wackenhut executives, several of whom have recently been fired.

Wackenhut said the guards were fired for just cause and the company denied wrongdoing. However, in the audit, Wackenhut admitted to about $3 million in over-billing.