At least 13 people were killed on Sunday in a bus crash in Thailand's Chiang Mai province, an official said.The bus plunged into a roadside pit while heading from Chiang Rai province to Chiang Mai, Xinhua reported.The dead include 12 Malaysian tourists and one Thai tour guide, the office of the Chinese consul general in Chiang Mai said.Twelve others were injured in the accident, he said.Police were investigating the cause of the accident.