The Bureau of Investigation is primarily responsible for investigating both reported crime and activities that could be criminal in nature or which may result in the commission of a crime. The B.O.I. is also responsible for processing and custody of property and evidence as well as the care and custody of prisoners held in the department holdover. Capt Randy Boden is the commander of the BOI and is responsible for the Investigations, Evidence, and Corrections Units. There are two Sergeants assigned to the BOI and they generally work opposite shifts insure supervision of the on duty personnel.

The Investigations Unit includes ten Detectives, each of which is highly trained and experienced. The Detectives work in teams of two and are assigned investigations by the type of crime or activity involved. Each Detective is a member of the Major Case Squad of Greater St. Louis, as are Capt Boden and Detective Sergeants Kyle Lewis and Dan Biermann.

In addition to participation in the Major Case Squad of Greater St. Louis, the Detectives support many regional investigative efforts including forensic computer crime, juvenile justice, auto theft, gangs, and narcotics. They meet regularly with other local law enforcement agencies to share intelligence and information on local crime trends that impact communities throughout the area.

The Evidence Technicians are responsible for the processing of all major crime scenes and the control and custody of all property and evidence secured by the department. They are equipped with state of the art equipment for gathering and processing evidence, including a complete digital photo and video laboratory, fully equipped evidence van and the latest in evidence detection and collection devices.

The Corrections Unit manages the department holdover that can confine a maximum of 27 prisoners. During 2011 the unit processed over 6,500 arrestees. Most prisoners held by the department are short term, awaiting transfer to other agencies or institutions, but many are held awaiting bond or appearances in the Municipal Court. To efficiently handle the flow of arrestees, the corrections unit relies upon cutting edge technology. Arrest photos are taken using high-speed video that captures the "mugshot" and transfers it to the central repository at the St. Louis County Justice Center. In similar fashion, fingerprints are now rolled on an inkless, computerized "Live Scan" system that insures the officer rolls acceptable impressions. The rolled prints are then electronically stored and transmitted to state and local AFIS fingerprint repositories. The IRIS and Live Scan system are integrated with the REJIS booking system to create a complete record on every arrest.