Tenant Will not Respond To Return Security Deposit

[John Noon]

I have a past tenant that will not respond to calls, voicemail, email, text so that we can return his deposit and mail. We also reached out to his attorney who has not had any success in contacting the tenant as well. I am not sure what our legal obligation is in this situation? We are in Oakland, California. Any advise or help would be much appreciated. Thanks.