According to local media , Hinrichs inquired about purchasing a large quantity of ammonium nitrate , the chief ingredient in the 1995 Oklahoma City bombing , at a local feed store two days before the explosion . An off-duty Norman police officer was in the store and overheard the conversation between the feed store owner and Hinrichs . The officer followed Hinrichs out of the store , checked Hinrichs ' license plate , and began an investigation when his shift started . That investigation was still in progress when the bomb went off . A computer check , however , showed no outstanding arrest warrants for Hinrichs and no criminal record
Oklahoma City bombing
Oklahoma City bombing
2005 University of Oklahoma bombing