Three drones used to spy on South Korea were built and flown by the North, the Defence Ministry in Seoul said on Friday, according to a news report.

The South Korean military has retrieved three small, unmanned aerial vehicles from areas near the tense border since late March, the Yonhap News Agency said.

One of them had reportedly taken photos of military installations and the presidential compound in Seoul.

“The drones have batteries with expiration dates inscribed in the North Korean dialect, and fingerprints unregistered in South Korea’s database were collected from one of them,” Yonhap quoted the ministry as saying.

It said the drones were made of polycarbonate, which is difficult for radars to detect.

The system was not sophisticated enough to send pictures in real time and the images were no better than those collected by Google using commercial satellites, the report said.

Parts from several countries, including South Korea, the U.S., Japan, China and the Czech Republic, were used to build the devices, Yonhap reported.

Company logos and serial numbers had been intentionally erased.

The Defence Ministry in Seoul announced the interim results of its investigation on Friday.

Defence and Foreign Affairs officials said if Pyongyang’s involvement is confirmed, South Korea will lodge a protest through the Onited Nations, UN, Yonhap reported.