Five people were killed and 51 others injured when a tour bus plunged down a roadside cliff in Northern Thailand early Monday, police said. The bus, en route from Udon Thani to Chiang Mai, veered off the main road and plunged down a roadside cliff in Phitsanulok about 2 a.m. local time. The five people who died included a foreign woman tourist and a Thai baby boy, Xinhua reported. It took over two hours for the rescue workers to extricate the survivors from the mangled wreckage and send them to hospitals. One of them is a foreign woman tourist with four-month pregnancy. The accident was being investigated. Police suspect it was caused by a brake problem.