Vigilance plays a role, but Shur suggested another factor may also play an important role.
Deputy marshals take the witness and the witness' family and move them to another area, Shur said.
"It's very unusual, very unusual for a student to withdraw from our program, " Dean Barry Shur told reporters.
Most witnesses who enter the program are not law-abiding citizens, according to Shur.
Though all applicants undergo background checks, Shur said he was aware of no program that requires that applicants undergo a psychiatric evaluation.
In an interview with CNN, Gerald Shur, who created the program, offered insight into the inner workings of this secretive government service.
Shur described an effective way of choosing where to relocate a witness.
Shur said his work protecting witnesses put his family's life in danger.
While witnesses and their family members are free to return to their former identities, they do so at their own peril, Shur said.
"They might say they were legitimate businessmen, but they would bribe people, " Shur said, recalling the case of an engineer who bought off a mayor.
Shur was an attorney in what was then the Justice Department's Organized Crime and Racketeering Section in the 1960s when he developed the idea to protect witnesses.
Entrance to the program is highly competitive: It generally has 10 applicants for each of the five or six slots that open in any given year, Shur said.
Shur recalled two instances when witnesses refused protection.
"It occurred to me that we have to have a way that if a guy is in danger because of his testimony, we have to get him out of there, and in a second, " said Shur, 79, who is now retired.
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