A supporters' group installed a poster on an advertising board outside Elland Road calling for the Italian to go earlier toady.
The advert features a cartoon image of Cellino under the heading 'Time to go Massimo' with various quotes from the former Cagliari owner underneath.
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The letter was sent to City Ads Yorkshire, the advertising company who put the advert up, but Ward Hadaway have also asked them to reveal the details of the people who organised/paid for the poster.
Cellino bought Leeds in 2014, but his time at the club has been overshadowed by an a number of controversies.
Most recently he angered supporters in December by imposing a 'pie tax' at Elland Road, raising ticket prices in the South Stand by £5 in exchange for food and drink vouchers, but he has since claimed it was just a joke.