The 30-year-old brother of Sam Serey, the exiled president of the Khmer National Liberation Front—a group branded as terrorists by the government—died in prison on Wednesday night from a heart attack, officials said on Thursday.
Mr. Serey cast immediate doubt over the official explanation and said he believed his brother, who was serving a seven-year sentence in Prey Sar prison and was recently named in Mr. Serey’s “government-in-exile,” may have died as a result of his dissidence.
“This is very sad for me. My younger brother Yean Yoeurb died in prison last night,” Mr. Serey said in an email from his home in Denmark on Thursday morning.
Yean Yoeurb was arrested in Thailand in 2013 along with six others from the group and sentenced the following year for plotting to overthrow the government. He was named the vice minister of economics and finance in his brother’s government-in-exile in October.
“They accused him of treason. I love my younger brother and he was killed by the intention of Hun Sen’s regime and their master,” Mr. Serey said.
Officials contacted on Thursday said Yean Yoeurb died after a heart attack.
“I received the name of a prisoner from Prey Sar prison, Yean Yoeurb, and he died because of a heart attack,” said Be Tea Leng, deputy director-general of the Interior Ministry’s general department of prisons.
Kim Sarin, another deputy director-general of the prisons department, also confirmed that a prisoner died of a heart attack at Prey Sar on Wednesday night.
Officials from Prey Sar could not be reached for comment.
Earlier this month, 11 more KNLF members were handed five- to nine-year prison sentences for plans they said amounted to a peaceful protest outside the Vietnamese Embassy in Phnom Penh in 2014, but which authorities claimed was intended to sow violence and disorder.
Mr. Serey was also convicted in 2014 for plotting to overthrow the government.