  Intracluster stars are believed to be unbound from their progenitor galaxies
and diffused throughout the galaxy cluster, creating intracluster light (ICL).
However, when and how these stars form are still in debate. To directly
constrain the origin, one powerful method is to study clusters at the epoch
when mature galaxy clusters began to appear. We report measurements of the
spatial distribution, color, and quantity of diffuse intracluster stars for a
massive galaxy cluster at a redshift of 1.24. This is the most distant galaxy
cluster to date for which those three properties of the ICL have been
quantified simultaneously. Our detection of the significant ICL fraction in
this unprecedentedly high redshift regime strongly indicates that intracluster
stars, contrary to most previous studies, might have formed during a short
period and early in the history of the Virgo-like massive cluster formation and
might be concurrent with the formation of the brightest cluster galaxy.
