Crowds and participants attending the America's Cup yachting competition in New Zealand are being warned to take safety precautions after a terror alert. Counter-terrorism police are investigating three threatening letters, including one containing cyanide, which were sent to the US Embassy and the British and Australian High Commissions in the capital, Wellington. Assistant Police Commissioner for Counter-terrorism, John White, said the identical letters were intercepted and "secured" at the Auckland Mail Centre after staff noticed some white powder. "A small quantity of cyanide (was) in one of the letters," White said. "It's quite a serious situation and we wouldn't want to take it lightly," he said.