In May 1863 the severe manpower shortage forced the War Department to approve the organization of additional black regiments led by white officers. The units were designated as the United States Colored Troops (USCT). The USCT constituted 13 percent of the Union Army. The 54th and 55th Massachusetts regiments were excluded from this designation. Over thirty such regiments were in place by the mid-1860s. After the USCT regiments were established, black Americans fought and died in all major actions during the Civil War. Usually their pay was much less than that of white soldiers. Blacks in some units, however, refused to accept the lesser pay. When black Union soldiers fell into Confederate hands, they were enslaved, re-enslaved, or executed, thus breaching the land warfare standards in place. One of the most brutal acts against black soldiers was the Fort Pillow Massacre, which occurred at Fort Pillow, Tennessee, on April 24, 1864. Even though the Union forces had surrendered, Confederate troops massacred the Union troops, killing more than three hundred African Americans, including fleeing civilians. Confederate leader General Nathan Bedford Forrest was identified as the responsible person in the act; he became a principal founder of the Ku Klux Klan after the war ended.