What caused the United States Congress to pass an act designed to economically isolate South Africa in 1986?

The Comprehensive Anti-Apartheid Act of 1986[1] was a law enacted by the United States Congress. The law imposed sanctions against South Africa and stated five preconditions for lifting the sanctions that would essentially end the system of apartheid. The sanctions were repealed in July 1991 after South Africa took steps towards meeting the preconditions of the act.