Please answer a question about the following article about Charleston, South Carolina:

Violent incidents occurred throughout the Piedmont of the state as white insurgents struggled to maintain white supremacy in the face of social changes after the war and granting of citizenship to freedmen by federal constitutional amendments. After former Confederates were allowed to vote again, election campaigns from 1872 on were marked by violent intimidation of blacks and Republicans by white Democratic paramilitary groups, known as the Red Shirts. Violent incidents took place in Charleston on King Street in September 6 and in nearby Cainhoy on October 15, both in association with political meetings before the 1876 election. The Cainhoy incident was the only one statewide in which more whites were killed than blacks. The Red Shirts were instrumental in suppressing the black Republican vote in some areas in 1876 and narrowly electing Wade Hampton as governor, and taking back control of the state legislature. Another riot occurred in Charleston the day after the election, when a prominent Republican leader was mistakenly reported killed.

What granted Freeman were American citizenship?
federal constitutional amendments.