Kim Jong Un said he’s “very sorry” the “unsavory incident” took place.

SEOUL, South Korea — North Korea sent an apology letter to South Korea’s presidential office on Friday explaining what happened in their waters when North Korean troops shot and killed a South Korean official who drifted into their waters earlier this week.

The letter from North’s Central Committee’s United Front Department also said that North Korean leader Kim Jong Un said he is “very sorry” that an “unexpected and unsavory incident” took place while not being able to help fellow South Korean people who are already going through tough times fighting the threat of a vicious virus.

“It’s extremely unusual for North Korea to issue a statement of regret so fast,” Byun Sang-Jung, a researcher at Institute for National Security Strategy, told ABC News. “North Korea felt the need for South Korea to calm down, seeing that not only the public is infuriated, but the presidential office issued a statement condemning North Korea.”

North Korean soldiers discovered a 47-year-old civil servant clutching on a floating device, questioned him from a distance, fired shots to kill, then burnt the corpse, South Korea’s Defense Ministry announced on Thursday based on their intelligence gatherings. South Korea strongly criticized the shooting calling it a “brutal act” and demanded explanation and punishment for the persons in charge.

According to North’s explanation on Friday, North’s army troops in charge of waters patrol spotted an unidentified man atop a flotsam. When asked to identify himself, he murmured at first and soon stopped answering. As he remained silent and not adhering to the patrol’s demand, the patrol shot two blanks. North’s troops fired towards the intruder from 130 feet when it came to a situation suggesting that he attempted to flee. After checking that the intruder had disappeared while “much volume of blood” was spotted, they set the remaining flotsam on fire.

In the very unusually long and detailed explanation, which North Korea rarely issued over any conflicts in the past, the North also expressed “big regret” that the South Korean military accused them on Thursday of committing an atrocious act without proof and prior to requesting an explanation.

ABC News’ Hakyung Kate Lee, Aaron Kwon and Haley HyunJoo Yang contributed to this report.