The National Intelligence Service is investigating shocking allegations that a leftist lawmaker and his associates formed an anti-state underground organization and plotted an armed revolt in support of North Korea. On Wednesday, NIS agents raided the offices and homes of Rep. Lee Seok-ki from the Unified Progressive Party and nine others affiliated with the UPP or other left-leaning political and labor groups. Three members of the minor progressive party were detained.

The lawmaker, who went into hiding upon the raid by the intelligence agency, showed up at a party meeting Thursday and insisted all charges against him had been fabricated. The legislator is alleged to have played a leading role in forming an underground group called the “Revolutionary Organization” before he was elected to the National Assembly last year, and instructed its members in May to secure firearms to hit infrastructure facilities if a clash with North Korea broke out.

The charges against Lee and his leftist colleagues are astounding, but they must be subject to thorough and objective investigation.

The raids sparked an angry reaction from the UPP, which accused President Park Geun-hye’s conservative government of reenacting the “Yushin dictatorship,” referring to the authoritarian rule of her late father Park Chung-hee in the 1970s. The party said in a statement it would fight against what it termed as an attempt by the presidential office and the intelligence agency to isolate and annihilate progressive forces.

But the grave charges against Lee and other UPP members cannot simply be set aside as politically motivated, as the court issued warrants to search premises and arrest suspects. The more reasonable judgment at this moment is that the NIS has secured some concrete evidence that could prove the suspicions.

The lawmaker, who was convicted of anti-state activities in 2003, should first cooperate with the investigation to clear the accusations against him, if they have all been fabricated, as he claims. The main political parties also need to refrain from using the issue of public security for partisan purposes.

The investigation is poised to become a double-edged sword for the intelligence agency, which is dogged by allegations that it meddled in last year’s presidential election. If the investigation is completed successfully, it would help the NIS deflect mounting pressure for drastic reforms. A failure to prove the rebellion charges would have catastrophic consequences for the agency, and deal a severe political blow to Park.