Investigators on Thursday raided the home and office of Ro Su-hui, a key member of a pro-North Korea group who illegally traveled in the communist state for more than three months, as part of their probe into his unauthorized visit.

Ro, 68, vice chairman of the Pan-Korean Alliance for Reunification, was arrested on charges of breaching the National Security Law upon his return to Seoul later in the day through the border village of Panmunjom. He will be questioned about his activities in the North.

The law bans South Koreans from visiting the North without prior approval and from honoring or supporting the autocratic regime. He faces up to 10 years in prison if convicted.

Ro sneaked into the North on March 24 via Beijing without the South Korean government’s permission to attend a memorial service that marked the 100th day of the death of the longtime leader Kim Jong-il.

The search and seizure was carried out at the organization’s office in western Seoul and the homes of Ro and another senior PKAR official.

The unidentified official was also arrested Thursday morning on charges of having joined an anti-national group and involvement in Ro’s illicit visit.

The alliance lambasted Seoul for trying to use Ro’s return “as a pretext for eradicating progressive pro-reform forces” with a December presidential poll months away.

The PKAR opened its South Korean unit in 1995. It is categorized as an anti-national organization by the Supreme Court for its campaign for the federal unification of two Koreas, nullification of the anti-communist National Security Law and withdrawal of U.S. troops here.

The probe and arrests are expected to further derail cross-border relations as the North threatened it would not “accept any suppression on Ro.” Seoul vowed stern punishment.

The South did not send any representatives to the senior Kim’s funeral and allowed for only two private delegations, a decision strongly condemned by Pyongyang.

During his 104-day stay, Ro paid tribute to the Kim dynasty at memorials, visited scores of tourist sites and had an interview with the KCNA, in which he called the late leader’s death “the greatest sorrow of the people.”

On Wednesday, he met with Kim Young-nam, the North’s ceremonial head of state believed to be second in line to incumbent leader Kim Jong-un, in a “compatriotic atmosphere,” the North’s official Korean Central News Agency said.

By Shin Hyon-hee (heeshin@heraldcorp.com)