A St. Paul police officer stepping out of a Bureau of Criminal Apprehension van.Credit...Aaron Lavinsky/Star Tribune, via Associated Press The second witness to take the stand in the trial of Derek Chauvin on Wednesday was a special agent with the Minnesota Bureau of Criminal Apprehension. Other employees of the agency are also expected to testify. So, what is it? The Bureau of Criminal Apprehension is a state police agency with a range of responsibilities, from investigating murders to testing specimens at a laboratory. Since 2014, it has also investigated all police killings by the Minneapolis Police Department and other “critical incidents,” such as when someone dies in custody.