Federal prosecutors have documented at least 350 instances of faulty background investigations done by private contractors and special agents for the U.S. Office of Personnel Management in recent years, illustrating what some lawmakers call systemic weaknesses in the granting of federal security clearances.

Reuters calculated the total by reviewing court documents and press releases from prosecutors for 21 cases resulting in convictions that involved the making of false statements from December 2004 to March 2012.

These are the cases government officials have cited to assert that action is taken against investigators who falsely claim to have reviewed records or done interviews for background checks submitted to OPM. Not all the cases identified a specific number of fabrications.

The 350 falsified reports represent only a small percentage of the number of background investigations conducted each year, either by OPM’s own investigators or a handful of private contractors it uses for most of the work.