Leading the modern investigations at Copan in the early 20th Century was the Carnegie Institute of Washington with famous Mayanist Sylvanus Griswold Morley as head of research at Copan from 1910 to 1919. He focused on trying to interpret the glyphs, so one may rightfully say he was the first one to conduct epighraphic work at Copan and broadened the perspectives of study on the ancient Maya. When his field notes & reports were finally compiled and had been the object of deep analysis he published "The Inscriptions at Copan" (Carnegie Institution of Washington, Publication 219) which is considered to be the model to follow for all subsequent archaeological studies on the ancient Maya.

Morley's studies of the many inscriptions at Copan, funded by Carnegie Institute, resulted in a deep revision of current theories. Morley questioned the validity of conclusions about the nature of social behaviour among Maya society. It was sustained at the time that the Maya were a theocratic society and that their writing's sole subject was divine in nature. Among his many observations to cause this revision was the recurring appearance of the date 9.16.12.5.17, or 6 Caban 10 Mol (which is July 2nd, A.D. 763 in the Gregorian calendar). About this date he says, "[this date] probably represents a real historical or astronomical event." Recent studies have revealed that it was the date when Copan's 16th and last ruler ascended the throne: Yax Pasaj Chan Yoaat.

He also noticed there was a date acknowledged in both Copan and the smaller site of Quirigua, 50 kilometers away. He concluded that this was probably a very important date which made part of the history shared by both ciites. Recent studies have revealed that on this date 9.15.6.14.6 or 6 Cimi 4 Sek (which was April 29th 738 A.D. in the Gregorian calendar), Copan's 16th ruler Waxklajuun Ub'aah K'awiil died. How it happened exactly is still unclear, as the account of the event varies slightly from one site to the other. Copan's account tells of a noble “flint & shield death” (meaning he died honorably in combat) while Quirigua's account tells a different story of his death. He may have been beheaded in Quirigua, where the glories of K'ak' Tiliw Chan Yoaat are celebrated and narrated on the tallest stelae in the Maya World. Waxklajuun Ub'aah K'awiil of Copan, who held the most artistically and architecturally prolific reign in Copan, was killed by his vasal in Quirigua.