The state spy agency on Friday referred an opposition lawmaker imprisoned on charges of plotting to overthrow the government to the prosecution for further questioning, officials said.

Rep. Lee Seok-ki of the minor opposition Unified Progressive Party was arrested by the National Intelligence Service (NIS) last week after the parliament voted in an overwhelming majority to allow his detainment.

He is accused of leading a secret organization called Revolutionary Organization with suspected close ties to North Korea and plotting to destroy key infrastructure facilities in the South in the event of an inter-Korean war including communication lines and railways.

Wrapping up its one-week investigation into Lee, the NIS earlier in the day sent the case to the Suwon District Prosecutors' Office for a further probe, according to prosecutors in the city just south of Seoul.

"We just began investigation into Lee in earnest today," said Cha Kyung-hwan, a prosecutor at the office. "After up to 20 days of investigation, we will decide whether to indict him."

The office earlier set up an investigative team comprising seven prosecutors in charge of the treason case.

The prosecution plans to question him on the nature of the secret organization, Lee's role there and their detailed planning for the alleged plot, according to the officials.

The spy agency handed over relevant data it has accumulated on the case to the prosecution that judicial authorities say would add momentum to the probe.

Some, however, raised skepticism over whether they would be able to secure hard evidence enough to indict Lee, as he vowed to refuse statement as he did during the spy agency probe.(Yonhap News)