Investigators raided South Korea's spy agency on Tuesday as they probed possible government links to a drone shot down over North Korea earlier this year.
Pyongyang accused Seoul of flying a drone into Kaesong in January, releasing images that purported to show debris from the downed aircraft.
Seoul initially denied the government was involved, with President Lee Jae Myung saying it would be akin to "firing a shot into the North".
But authorities said on Tuesday they were investigating three active-duty soldiers and one spy agency employee.
Investigators from a joint military-police task force raided 18 locations of interest, including the Defense Intelligence Command and the National Intelligence Service.
"The task force said it will thoroughly establish the truth behind the drone incident through analysis of seized materials and a rigorous investigation of the suspects," a statement read.
Three civilians have already been charged for their alleged role in the drone scandal.
One of them has publicly claimed responsibility, saying he acted to detect radiation levels from North Korea's Pyongsan uranium processing facility.
Disgraced ex-president Yoon Suk Yeol is currently standing trial on charges he illegally sent drones into North Korea to help create the pretext for declaring martial law in late 2024.
His attempt to overturn civilian rule failed, and Yoon was impeached and ousted from office in April last year.
Provocation and propoganda
Prosecutors have accused Yoon of instructing Seoul's military to fly drones over Pyongyang and distribute anti-North leaflets in an attempt to provoke a response.
They said Yoon and others "conspired to create conditions that would allow the declaration of emergency martial law".
North Korea said last year it had proven that the South flew drones to drop propaganda leaflets over its capital.
Lee said in December that he felt an apology was due to North Korea over his predecessor's alleged order to send drones.
"I feel I should apologise, but I hesitate to say it out loud," he said at the time.
"I worry that if I do, it could be used as fodder for ideological battles or accusations of being pro-North," he added.
Lee has taken steps to ease tensions since taking office, including removing propaganda loudspeakers along the border.
Pyongyang unveiled new attack drones in August 2024, with experts saying the capability may be attributable to the country's budding alliance with Moscow.
North Korea's leader Kim Jong Un at the time ordered the "mass production" of attack drones, with analysts saying they may be designed to carry explosives and deliberately crash into enemy targets.
North Korea has previously sent trash-filled balloons over the South in what it called retaliation for activists in the South floating anti-regime propaganda missives northwards.
