Shortly after being elected to the National Assembly on proportional representation last year, Lee Seok-ki of the minor Unified Progressive Party refused to sing South Korea’s national anthem at public ceremonies. Instead, he proposed to replace it with “Arirang,” a traditional folk song popular in both Koreas.

The suggestion, though discomfiting, could have been dismissed as a condonable aberration, had it come from any guileless lawmaker. But it vexed many, who wondered in which Lee’s loyalty lay: the South or the North.

The reason was that the proposal was made by a lawmaker who, in 2003, had been sentenced to two years and six months in prison for attempting to build North Korean-style socialism in the South. He had been freed several months later by a presidential pardon.

Now Lee finds himself under suspicion of plotting an insurrection by organizing a secret revolutionary group. The National Intelligence Service reportedly claims he ordered the group in May last year to launch a general strike in support of North Korea and an armed rebellion against the South Korean government when a critical moment came. The agency is said to hold a transcript of his order in its possession.

The allegations are truly unsettling. If they prove to be true, Lee will be the first member of the National Assembly to be found to have attempted an insurrection ― the crime of rising in arms or open rebellion against the South Korean government.

Under the supervision of the prosecution, which had been issued warrants by the court, the intelligence agency conducted search and seizure at 18 places, including Lee’s home and office, on Wednesday. The prosecution detained three of Lee’s associates, also on charges of plotting an armed rebellion.

In all criminal cases, the burden of proof lies with the prosecution, not with defendants. And by extension, it lies with the intelligence agency in the case involving Lee and his associates. With Lee’s supporters already pointing this out, the intelligence agency will be dealt a deadly blow if it fails to establish facts and produce evidence against the accused.

Moreover, detractors suspect the intelligence agency is using the case as a distraction at a time when it is in crisis. Indeed, pressure is mounting on political parties to rein in the powerful agency, which is accused of breaching the law by meddling in the presidential election last year.

The agency undoubtedly has more than reasonable suspicion, given that the court issued search and seizure warrants. Still, the case will not prevail unless it is backed by objective and specific evidence. One legal expert said that a transcribed order to “secure firearms” alone would not be enough to send Lee to prison.

If it wishes to prove Lee guilty of attempting an insurrection, as it claims he is, the intelligence agency will have to leave no stone unturned in its investigation. Whether it obtains watertight evidence will make or break the case.