According to Politico, a new report exposes the shortcomings with the FBI’s surveillance capabilities when it comes to keeping tabs on potential terrorism suspects within the United States. The bureau doesn’t have the proper resources to watch, or properly investigate, all of the names on the suspect list. This has left a few holes in the system through which suspects have fallen and committed terrorist acts.

Omar Mateen was investigated on two previous occasions by the FBI before his attack on the Orlando nightclub. Mateen was cleared both times, and he was never placed under surveillance. In other cases, the shooters of the Fort Hood and Little Rock military attacks, as well as one of the Boston Marathon bombers, were all investigated at one point by the FBI before they committed their acts of terror.

Politico reports there are between 300 and 500 people on the terrorist watch list and roughly 16,000 on the no-fly list (about 500 Americans are on that list). It would be impossible for the bureau to keep tabs on them all, which means the bureau’s 13,000 agents must often rely on their gut when investigating a potential terror suspect.