Story highlights South Korea's National Intelligence Service says it will investigate whether the student violated national security law

Won-moon Joo has said he crossed into North Korea to make a statement

A permanent resident of the U.S., he took a semester off from NYU to travel

Seoul, South Korea (CNN) North Korea has released Won-moon Joo, a South Korean student at New York University whom it detained in April.

The student was handed over to South Korean officials at the border Monday afternoon, Seoul said. North Korea said it had deported Joo as a "humanitarian measure."

In an interview with CNN in Pyongyang in May, Joo said he had been arrested by soldiers after crossing into North Korea from China to make a statement.

"I thought that by my entrance to the DPRK -- illegally, I acknowledge -- I thought that some great event could happen and hopefully that event could have a good effect on the relations" between the two Koreas, Joo told CNN's Will Ripley, using an abbreviation of North Korea's official name, the Democratic People's Republic of Korea.

Joo, who is in his early 20s, said at the time that he wasn't sure what kind of great event could happen as a result of his actions.

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