MIAMI -- Miami-Dade and federal investigators raided the headquarters Friday night of one of the county's largest government contractors.

NBC 6 was the first to report in May that Wackenhut Security is under a criminal investigation for overbilling taxpayers millions of dollars, money for work on transit and the downtown juvenile center.

NBC 6 camera's filmed public corruption investigators and police removing boxes filled with documents from Wackenhut's Miami-Dade headquarters on Blue Lagoon Drive.

Investigators were there for several hours and were being assisted by top Wackenhut executives.

Wackenhut has repeatedly declined to be interviewed, but said in a statement that the company was cooperating with authorities.

"The Wackenhut Corporation ('Wackenhut') continues to cooperate with Miami-Dade County ('MDC'), and voluntarily provided MDC additional records and documents yesterday to assist and facilitate MDC’s investigation and audit of Wackenhut’s performance under its security contract with the Miami Dade Transit Authority," said Drew Levine, president of the Security Services Division. "Wackenhut is proud of its service and performance under its contracts with Miami-Dade County and is very confident that after a thorough investigation the County will conclude that Wackenhut acted properly and performed its responsibilities under the contract in a highly professional and responsible manner."

The company has previously denied overbilling taxpayers.

Miami-Dade County is nearing completion of an audit of Wackenhut's billing practices. The preliminary audit found serious discrepancies.