In jurisdictions throughout United States, thousands of sexual assault kits (SAKs) (also termed “rape kits”) have not been submitted by the police for forensic DNA testing. DNA evidence may be helpful to sexual assault investigations and prosecutions by identifying offenders, revealing serial offenders through DNA matches across cases, and exonerating those who have been wrongly accused, so it is important to understand why police are not utilizing this evidence. In this study, we applied focal concerns theory to understand discretionary practices in rape kit testing. We conducted a three‐year ethnography in one city that had large numbers of untested SAKs—Detroit, Michigan—to understand why thousands of SAKs collected between 1980 and 2009 were never submitted by the police for forensic DNA testing. Drawing upon observational, interview, and archival data, we found that while practical concerns regarding resources available for forensic analysis were clearly a factor, as Detroit did not have the funding or staffing to test all SAKs and investigate all reported rapes, focal concerns regarding victim credibility and victim cooperation were more influential in explaining why rape kits were not tested. Implications for the criminal justice system response to sexual assault and rape kit testing legislation are examined.