Prosecutors have started questioning Rep. Lee Seok-ki of the United Progressive Party after arresting him Thursday on charges of conspiring to overthrow the Seoul government and forming an anti-state underground organization.

They were able to detain the leftist legislator as the National Assembly approved his arrest Wednesday. The motion passed by an overwhelming margin, 258-14, showing that lawmakers took Lee’s alleged pro-North Korean activities very seriously.

Now prosecutors and investigators of the National Intelligence Service face the tough task of substantiating the rebellion agitation and conspiracy allegations they brought against Lee and his followers.

It may take more than the transcripts of the secret meetings Lee had with his followers to prove them guilty.

Most legal experts say the transcripts provide indisputable evidence that Lee incited the members of the underground organization, dubbed the “Revolutionary Organization,” to stage an armed revolt.

Participants in the meetings discussed, under Lee’s instruction, how to destroy infrastructure facilities such as oil storage tanks, power transmission towers and communication centers.

But some legal scholars say that prosecutors will have to present other evidence as the transcripts tell little about the organization’s true nature and whether its members systematically followed up on Lee’s instructions.

In this regard, investigators will first have to show the veiled underground organization in its true light and find out the role that each of its members played.

They are also required to trace the organization’s alleged connections with the North Korean espionage agencies and investigate its ties with other underground organizations that still remain hidden.

While prosecutors and NIS investigators interrogate Lee, lawmakers on the Assembly’s Special Committee on Ethics need to do the homework that they have kept delaying ― a review of the qualifications of Lee and another UPP lawmaker, Kim Jae-yeon.

The two UPP lawmakers are suspected of having been chosen as the party’s proportional representative candidates through a rigged primary, which was held before the general election in April last year.

After the election, the UPP conducted its own investigation into the primary and found that it was fraud-ridden. The party’s leadership sought to invalidate the primary outcome and pressured Lee and Kim to give up their parliamentary seats. But the two dug in their heels.

As the UPP could not force the two lawmakers to step down, the ruling Saenuri Party and the main opposition Democratic Party agreed to refer them to the Assembly’s ethics committee to see if they were qualified as legislators.

After a long delay, the two parties finally referred them to the committee in March. But no progress has since been made. Earlier this week, the two parties agreed to convene the committee next week to start the review process.

They should have done so earlier, when they could have nipped the whole affair in the bud, but it is better late than never. Lawmakers on the committee should speed up the screening process to expel Lee from the Assembly.

To ensure that the committee operates more efficiently going forward, bold reform is needed. Currently, the committee consists of 15 legislators from the two major parties. As such, it is bound to be dictated by partisan interests.

To change this, the committee should be staffed with external experts, including those from civic groups, the judiciary and the academic community.