Chess Makes a Statement

But what's the statement?

The caption read,

4/17/72 -- Attica, NY : Some of the inmates at the Attica Correctional Facility play chess -- but with unseen opponents. This photograph by Cornell Capa is part of a photographic essay he produced for the McKay Commission, which is investigating the September 1971 uprising at the state prison. The commission made public Capa's report 4/17, as it resumed public hearings in New York. (UPI)

For more about the uprising, see Attica Prison Riot (Wikipedia).