One person who posted pro-North Korea online messages believed he would be treated better in North Korea after finding that his pen name had been reported by the North's main Rodong Sinmun newspaper, the official said.
The six men were separately detained in North Korea for 14-45 months for questioning. One said he was constantly held in solitary confinement, while another said he didn't receive medical treatment for a kidney stone, according to the South Korean official.
South Korean authorities are seeking a warrant to formally arrest the six men, aged between 27 and 67, for a more thorough investigation, the official said. In South Korea, anyone who defects to North Korea can be punished by up to 10 years in prison under the country's anti-North security law.
The results of a preliminary investigation were provided to South Korea media, and another South Korean official confirmed the substance of the reports.
Seoul estimates that hundreds of South Koreans have been kidnapped and detained by North Korea since the Korean War ended with an armistice, not a peace treaty.