 His efforts, however, were thwarted when the Boipatong massacre caused a resurgence of international pressure against South Africa, leading to a weaker position at the negotiation tables for the National party.In 1992, de Klerk held a whites-only referendum on ending apartheid, with the result being an overwhelming "yes" vote to continue negotiations to end apartheid.Nelson Mandela was distrustful of the role played by de Klerk in the negotiations, particularly as he believed that de Klerk was knowledgeable about 'third force' attempts to foment violence in the country and destabilize the negotiations.