Here in Chicago, it is difficult to get a handle on exactly the role federal authorities play or how often they are asked to investigate questionable police shootings. Cook County prosecutors do not keep statistics on how often they seek assistance from the FBI or the U.S. attorney's office. The Independent Police Review Authority, the city agency that investigated more than 400 shootings by Chicago police officers since it was created in fall 2007, only began this year to track cases referred for possible federal investigation.