According to the indictment, Fire Chief Marc Jordan took vials of Valium from Katy city ambulances and gave the drugs to someone else.
Sara Marie Kinney with the Harris County DA's office says the incident reportedly happened in January.
"We received at the DA's office, and some other law enforcement agencies received anonymous tips from several sources."
The charges against the 58-year-old Jordan include diverting a controlled substance and tampering with evidence.
"And the tampering charge comes from his cover-up into how those were taken. It's a very well-documented process for any type of controlled substance to be taken out of an ambulance."
Jordan became the director of Katy's emergency medical services in 2002. He became fire chief in 2005.
City officials say they are now conducting an administrative investigation, and that an acting chief is now running the department.