Not only is making employees work off the clock an illegal wage and hour practice, if the employees are physically prevented from leaving the premises, the employer may be criminally and civilly liable for unlawfully detaining people against their will. Encourage your granddaughter to seek the help of an employment law attorney as soon as possible.
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This is a violation of state wage laws. Employees must be paid for all hours in which they are under there employer's control. Locking employees in a facility and not letting then out certainly qualifies as compensable time.
The locking of the doors may be acceptable as safety precaution while money is being counted, but they are entitled to be paid for that time.
If you'd like to discuss, please feel free to call. Jeff Gold Gold, Benes, LLP 1854 Bellmore Ave Bellmore, NY 11710 Telephone -516.512.6333 Email - Jgold@goldbenes.com