PHNOM PENH (Reuters) - A Cambodian reporter investigating extortion at a Phnom Penh market was shot dead by a security guard at the market after an argument, the director of his newspaper said on Tuesday. Ou Sareoun, a reporter for the weekly Samleng Reas Khmer (Voice of the Cambodian People) newspaper, was killed on Monday, Ou Dina, the director of the newspaper said. The reporter was investigating illegal payments which guards allegedly demanded from market vendors when he got into an argument with some market security men, Ou Dina said. Police confirmed the shooting but declined to comment on the motive. They said they were investigating the killing and expected to arrest the gunman soon.