Hello Chief Inspector Cain here. The investigators should get the names of those being ripped off, match these names to the teller(s) used, they might find a pattern. Also, check those who are involved with customers making large withdrawals such as Managers who would usually have to sign off on such withdrawals.
No personal phones to be used during work, that is what breaks are for. Next step, get permission to bug the phone lines.
Chief inspector, you made some good points. Is joke to see if half the bank employees are guilty. Lol