Ask further details from your bank and particularly ask for the transactions which are market into lien (charge back) and crisis check with the receipts of your LocalBitcoins to find which transaction and buyer claimed the charge back.
Nowhere on this planet will the police investigating fraud tell you what is wrong with your account and what transactions are involved before the end of the investigation.
For example, here banks are actually forced to not give out any information, anyone telling their clients any bit of information that was requested by the police also faces charges.
But seizing funds without enough proof is also against the law right, I am not sure how a bank cannot say which of your transaction got charged back!