Allegations have been raised that the National Intelligence Service played a role in fabricating evidence against a North Korean defector being tried on charges of espionage.

Prosecutors launched a full-fledged probe into the allegations Friday, banning several NIS agents from leaving the country for their alleged involvement in the evidence forgery.

Now the central question is no longer whether Yoo Woo-seong, a 34-year-old defector from the North who served as a civil servant for the Seoul municipal government, is North Korean.

The attention has shifted to whether the state spy agency was really involved in forging the evidence. If the allegations turn out to be true, the NIS will suffer irreparable damage to its already tarnished reputation.

The allegations surfaced following the failed suicide attempt by a key witness in the Yoo case. A 61-year-old ethnic Korean with Chinese nationality, the witness is believed to have served as an undercover informant for the NIS.

The man attempted to kill himself after being questioned by prosecutors. He probably felt a sense of betrayal as the NIS failed to keep his identity under wraps.

He left a note claiming that the NIS had helped him fabricate Chinese immigration documents to prove Yoo had visited North Korea from China. The Chinese authorities confirmed that the documents had been forged.

He even told his sons to collect 10 million won from the NIS as the spy agency had not paid his expenses for producing the fake evidence.

NIS officials deny that they helped the informant forge the evidence. They claim innocence, asserting that had they known about the forgery, they would not have provided the documents to prosecutors to help indict Yoo.

But their claims are not convincing. The NIS should not attempt to cover up the allegations. It should realize that such an attempt would trigger uncontrollable backlash. The main opposition Democratic Party has already called for a special counsel probe to get to the bottom of the matter.

Prosecutors need to rush to establish the truth. The government needs to ensure that a thorough investigation is conducted into the case without leaving any doubts.

Failure to do so would cost the nation dearly. Last year, the National Assembly was frequently paralyzed due to a protracted political standoff over the spy agency’s alleged meddling in the 2012 presidential election. The nation cannot afford a repeat of that kind of political strife due to the ineffectual and corrupt spy agency.