According to Gerald Shur, who created the federal program, about 95% of witnesses in the program are "criminals". They may be intentional criminals, or people who are doing business with criminals, such as one engineer who bought off a mayor "'because that's how you do business in the city.' In his mind, he wasn't doing anything criminal," as Shur said. A witness who agrees to testify for the prosecution is generally eligible to join the program, which is entirely voluntary. Witnesses are permitted to leave the program and return to their original identities at any time, although this is discouraged by administrators.