The photographer who took the alarming photos of the Lincoln conspirators hanging from the gallows knew they were to be hanged before the conspirators did.

A military commission issued a pass to Alexander Gardner before the pubic or the condemed had been informed of the imminent hangings.

Gardner and another photographer arrived at the prison yard at 11 a.m. on July 7 and set up their cameras in two windows facing the scaffold.  The condemed were led to the scaffold around 1:15 p.m.

ag just after hanging

Source: The Lincoln Assassination Encyclopedia by Edward Steers Jr.