They should have shown the warrants, but ultimately if they had "no-knock" warrants then this would weaken any claim you have. Nevertheless, I advise you & your brother to schedule a consultation with a NYC Civil Rights lawyer to discuss the case in more detail.
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There are exceptions to requirement to have a warrant when they do search at home. As to brutally beating of your brother at the time of arrest without any reason and without him resisting arrest, his present criminal defense lawyer should be made aware of this and made aware of any injuries caused to him at the time of arrest.
Please note that the information presented is for general informational purposes. The information does not constitute legal advice and does not create attorney-client relationship.