South Korea arrests activist after he visits North Korea July 5, 2012 -- Updated 1441 GMT (2241 HKT) Roh Su-hui went to North Korea to mark the 100-day anniversary of Kim Jong Il's death He is likely to be charged under the National Security Law, a police official said Tensions remain high between the two countries (CNN) -- A South Korean left-wing activist was arrested Thursday for an unauthorized visit to North Korea, a South Korean national police official said. Roh Su-hui, a leader with an organization close to North Korea, was taken into custody as he crossed back into South Korea after three months, the official said. Roh visited Pyongyang to commemorate the 100th day since the death of the country's late leader, Kim Jong Il, according to KCNA, North Korea's official news agency. KCNA called his visit "a righteous deed in view of traditional beautiful manners" and urged the South not to arrest him. Nevertheless, he was immediately arrested for illegal entry and glorifying his country's political rival. He walked across the two countries' heavily guarded border in the demilitarized zone and was brought in for questioning, according to an official from the South's National Police Agency who refused to be named in line with Korean policy. So far, he has refused to speak about his visit, the official said. Authorities will be interrogating him and will likely charge him under the National Security Law (NSL) within two days, the official told CNN. The NSL was designed to protect the state against North Korean propaganda and to prosecute spies. It was used widely during the years of military dictatorship in the 1970s and '80s. Roh Su-hui's illegal entry to North Korea and his slandering of the South's government and praising of the North is an illegal act. Upon Roh Su-hui's return, he will be investigated on the purpose of his visit and his whereabouts in the North and will be strictly dealt with in according to relevant law," Unification Ministry spokeswoman Park Soo-jin said in a regular briefing Wednesday. North Korea claims that "the South is using Roh's return as a pretext for eliminating the progressive pro-reform forces." South Korea did not send any representatives to the North after Kim's death. Tension between the two countries remains high since North Korea's shelling attack on a South Korean island in 2010 and the sinking of a South Korean warship in a torpedo attack blamed on the North. April 16, 2012 -- Updated 0715 GMT (1515 HKT) For just a moment we can hardly believe what is happening. Kim Jong Un addresses the public -- something his father only did once during his leadership. April 16, 2012 -- Updated 0133 GMT (0933 HKT) CNN's Paula Hancocks looks at the differences between new North Korean leader Kim Jong Un and his late father's styles. April 13, 2012 -- Updated 0049 GMT (0849 HKT) CNN's John Vause talks to The Ploughshares Fund president Joseph Cirincione about North Korea's failed rocket launch. The cost of the failed rocket launch is estimated at $850 million -- nearly enough to feed about 80% of the impoverished nation for a year. April 13, 2012 -- Updated 0244 GMT (1044 HKT) Former U.S. Governor Bill Richardson tells CNN's Piers Morgan that the rocket launch was a cover for testing ballistic missile technology. The controversial launch will provide a rare opportunity to see how far North Korea has progressed with its long-range missile technology program. April 12, 2012 -- Updated 0836 GMT (1636 HKT) David Austin is one of the few outsiders who has seen firsthand how people live in the North Korean countryside, a population "lethargic" from malnutrition. April 9, 2012 -- Updated 1303 GMT (2103 HKT) Former U.S. Ambassador Christopher Hill says North Korea's determination to push ahead with a rocket launch masks potential splits at the very top. April 11, 2012 -- Updated 1801 GMT (0201 HKT) As the media is given access to this reclusive country ahead of a controversial rocket launch, CNN documents this rare opportunity in pictures. April 12, 2012 -- Updated 1452 GMT (2252 HKT) On the centenary of the birth of the man North Koreans call "Great Leader," Stan Grant witnesses the god-like status conferred on the country's founder.