Widening its probe into the scandal-ridden cyber command, military investigators have investigated all officials in charge of psychological warfare operations to look into their involvement in an alleged smear campaign, sources familiar with the matter said Thursday.

The defense ministry's cyber command has come under investigation after some of its officials were caught posting political messages against the opposition camp ahead of last December's presidential poll.

With suspicions lingering over whether the officials acted on orders to sway public opinion, military investigators have raided offices and questioned about 70-80 members belonging to the psychological warfare unit and its chief, a military source said, asking for anonymity citing the ongoing probe.

Tasked with conducting psychological warfare operations against North Korea's propaganda, the unit plays a key role in the 100-member command.

The director of the psychological warfare unit is accused of ordering his subordinates to post messages against the liberal camp ahead of last year's presidential vote, the source said, noting legal reviews are underway to issue warrants to arrest him.

Some cyber warfare officials posted or spread writings on social networking sites such as Twitter that were critical of the then opposition camp's presidential candidate Moon Jae-in in violation of their obligation to stay politically neutral, another official with knowledge of the matter said.

The defense ministry said it will announce the interim results of the investigation within this month, with some of them expecting to face an investigation by state prosecutors depending on their involvement.

"Currently, the ministry is conducting an investigation to press charges against those responsible and issue arrest warrants (against those involved in the case)," ministry spokesman Kim Min-seok said in a briefing.

The latest move comes as opposition lawmakers demand a special independent counsel into the cyber command, claiming its online smear campaign is part of a larger scheme that also involves the state spy agency.

On Thursday, Rep. Jin Sung-jun of the main opposition Democratic Party criticized the ongoing investigation for not getting to the essence of the real case, raising allegations that the warfare unit director may have acted on an order from upstairs to conduct the online smear campaign. (Yonhap News)