Cambodian twin brothers and environmental activists Chum Huor and Chum Huot were detained briefly by immigration officers on Friday while trying to cross the border into Vietnam, one of the two men told RFA's Khmer Service. The pair, who were traveling by motorbike, had been attempting a short cut through Vietnam to cross back into Cambodia to visit the Pak Nam temple in Kandal province's Koh Thom district, Chum Huot told RFA by phone. "They may have thought we were going there to stir up trouble over border issues at the temple," Chum Huor said. "We were the only two people who were stopped, questioned, and not allowed to cross the border. Other people could travel freely, and some didn't even need to show their identification to the authorities," he said. "This was very unjust," he added. After being held for three hours at the border, the two men were released after fixing their thumbprints to a document provided by authorities, but were not allowed to cross into Vietnam, he said. Also speaking to RFA, Kim Sarin--head of the refugees and immigrants department of Cambodia's Ministry of Interior-said the two brothers were stopped because they had failed to obtain passport stamps authorizing an earlier trip to Thailand. "It was illegal to leave and re-enter Cambodia without them," Kim Sarin said. "We won't allow them to leave Cambodia unless they produce proper immigration documents regarding their previous travels." Active in protests Chum Huor and Chum Huot, close friends of assassinated government critic Kem Ley, had fled Cambodia on July 14, four days after the pundit's murder and after they had posted criticisms of the government's investigation into the murder online. They have also been active participants in campaigns and protests to stop the building of the 260-megawatt Don Sahong hydropower dam being built along the Mekong River in southern Laos, less than a mile from the Cambodian border. Environmental activists say the dam will block transboundary fish migration routes and disrupt the livelihoods of villagers living near the dam and along the river and its tributaries. Chum Huor and Chum Huot are "prominent activists who are engaged in raising social issues related to the border and the environment," Am Sam Ath, technical adviser to the Cambodian rights group Licadho, told RFA on Friday. "They used to encounter similar[travel] restrictions when they tried to go to Laos to protest the construction of the dam," he said.