Two Chinese nationals were arrested on Thursday for filming U.S. jets at Osan Air Base in South Korea. This is the second time this week that the two were detained.

The Korea Times reported that the Gyeonggi Nambu Police Agency stated the U.S. Army had reported the two, whose identities were not revealed, after they spotted them taking pictures near the 51st Fighter Wing air base.

The two were apprehended by police and it was confirmed that they had been caught doing the same thing in the same place on Monday. The police, working with South Korea’s National Intelligence Service, Defense Counterintelligence Command, and other agencies, concluded that there were no signs of espionage eight hours after they had been detained.

The Times reported that a similar conclusion was reached after the incident on Wednesday. According to reports, the two men were released without charge after police determined that no laws had been violated, as they only took photos of aircraft in flight. According to South Korean law, taking pictures of aircraft outside designated security areas is not a crime.

It is not the first instance that Chinese nationals were caught photographing military installations in South Korea. Two Chinese teenagers were arrested earlier this month on suspicion of photographing fighter planes illegally during takeoff and landing using smartphones and cameras.

The police caught them in Suwon (Gyeonggi Province), home of the 10th Fighter Wing of the South Korean Air Force. The base is a vital part of defending airspace in the Seoul metro area. The authorities later found that the teens took thousands of pictures not only at Osan Air Base, but also four other major South Korean military bases and U.S. military installations, including Camp Humphreys, Pyeongtaek, and the South Korea Air Force Base in Cheongju in North Chungcheong Province.

The Times reported that police said one of the teens claimed his father worked for China’s Public Security Bureau. The Times reported that although no evidence of involvement by the Chinese government has been found, prosecution on espionage counts would be difficult.

The Times reported that espionage in South Korea is punishable with death, life in prison, or at least 7 years in jail – but only if it was committed for an enemy. And under South Korean law, this term applies exclusively to North Korea. To be recognized by law as an enemy country, a nation must be at war with South Korea. This condition only applies to North Korea. The same does not apply to espionage committed on behalf of China or any other country.