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Most likely Office Depot would not be responsible for your losses. They are generally not responsible for the unauthorized criminal acts of another person.
It is possible if Office Depot did something negligent or reckless, they could be responsible. For example, if the gunmen worked there or Office Depot failed to take some ordinary precautionary measure.
But most likely, they would not be responsible. Sorry. The responsible party is the one that performed the criminal act. If and when the gunmen are caught, they will have to pay restitution.