In December 1958 Ngo Dinh Nhu, Diem's younger brother and chief adviser, broached the idea of orchestrating a coup to overthrow Sihanouk. Nhu contacted Dap Chhuon, Sihanouk's Interior Minister, who was known for his pro-American sympathies, to prepare for the coup against his boss. Chhuon received covert financial and military assistance from Thailand, South Vietnam, and the CIA. In January 1959 Sihanouk learned of the coup plans through intermediaries who were in contact with Chhuon. The following month, Sihanouk sent the army to capture Chhuon, who was summarily executed as soon as he was captured, effectively ending the coup attempt. Sihanouk then accused South Vietnam and the United States of orchestrating the coup attempt. Six months later, on 31 August 1959, a small packaged lacquer gift fitted with a parcel bomb was delivered to the royal palace. Norodom Vakrivan, the chief of protocol, was killed instantly when he opened the package. Sihanouk's parents, Suramarit and Kossamak, were sitting in another room not far from Vakrivan. An investigation traced the origin of the parcel bomb to an American military base in Saigon. While Sihanouk publicly accused Ngo Dinh Nhu of masterminding the bomb attack, he secretly suspected that the US was also involved. The incident deepened his distrust of the US.