A man was stabbed to death in a brutal gangland attack in Southall, west London.
Mr X was among a string of suspects rounded up by police and detained at a station in the north of the capital.
Stuart Miller Solicitors was instructed when his family in Birmingham learned of his arrest.
The complex murder probe – which involved mobile phone data, CCTV analysis and scores of witness statements – saw six suspects charged over the killing.
Law firm director Mohammed Zeb stood side by side with Mr X as he faced three days of extensive police questioning.
His advice meant that despite multiple lengthy interviews and being detained under warrant, Mr X was released with no further action taken against him.
A spokesman said: “Stuart Miller Solicitors operates on a national basis and regularly receives instructions from across the country.
“Our director, Mohammed Zeb, ensured that Mr X received the best advice which resulted in no further action being taken against our client.”