Denied a military victory, the Constitutionalist rebels quickly had a Constitutionalist congress elect Caamaño president of the country. U.S. officials countered by backing General Antonio Imbert. On May 7, Imbert was sworn in as president of the Government of National Reconstruction. The next step in the stabilization process, as envisioned by Washington and the OAS, was to arrange an agreement between President Caamaño and President Imbert to form a provisional government committed to early elections. However, Caamaño refused to meet with Imbert until several of the Loyalist officers, including Wessin y Wessin, were made to leave the country. On 13 May General Imbert launched an eight-day offensive to eliminate rebel resistance north of the line of communications. During the attack, U.S. troops shot down one of the new government's five P-51 Mustangs when it accidentally strafed their position. Imbert's forces took the northern part of the capital, destroying many buildings and killing many black civilians. The United Nations dispatched a human rights team to investigate alleged atrocities.