SEOUL — Three crashed drones were launched from North Korea for spying purposes and had been programmed to fly over South Korean military installations, the Ministry of National Defense said Thursday.

An MND statement said the “smoking gun” of a joint South Korean-U.S. investigation was an analysis of the drones’ flight plans, which showed that all were launched from and intended to return to North Korea. South Korea’s Yonhap News reported that one drone crashed due to an engine problem, while the other two ran out of fuel.

Baek Seung-joo, vice minister of national defense, told reporters that the drones were trying to gain visual intelligence of South Korea and test whether its air defense system could detect small objects.

The rudimentary blue drones, which were all similar in size and design, were equipped with cameras and parachutes and, according to the MND, pose a new military threat, even as North Korea is believed to be preparing for its fourth nuclear test.

Baek said the drones did not have the capability to carry a significant load of weapons or the software to provide a live feed to a ground contact, indicating the North is in the early stages of developing unmanned aerial vehicle technology.

“We see this as the beginning stage of ground-level work of UAVs,” he said.

However, Baek cited North Korea’s “irrational” and “unpredictable” nature in saying drones could pose a threat if Pyongyang tries to load some amount of weaponry on them.

One drone, found March 24 in the border city of Paju, was launched about three miles from the North’s town of Kaesong, home to a jointly run industrial complex. Yonhap reported that it had taken photos of the Blue House, the president’s office in Seoul, as well as other installations.

Another drone was recovered a week later on the border island of Baenynyeong and was launched from about 17 kilometers southwest of Haeju. Yonhap report that it had taken photos of troops on two South Korean islands.

A third drone was found last fall on a mountain in Samcheok, in the eastern part of the country, by area residents, though South Korea’s military did not announce that discovery until April 6. That drone was launched from about 10.5 miles east of Pyongyang, but no photographs were found on it.

An MND spokesman said the joint investigation team, which formed April 14, included 10 Americans and 15 Koreans. He described the Americans as drone experts with a wide range of experience in computer software and the technology of small aircraft.

Last month, South Korea issued an interim investigation citing a “striking similarity” in appearance to drone models shown in North Korean media footage in 2012 and 2013, leading investigators to conclude that they were of North Korean origin.

Six fingerprints not traceable in South Korea’s national database were also found on two of the drones.

The drones’ ability to penetrate South Korean airspace without notice has raised questions about the country’s air defense. The MND vowed Thursday to “respond forcefully” to future drone provocations and said efforts are under way to enhance its ability to monitor and down UAVs.

Yonhap reported that Seoul is trying to buy about 10 low-altitude radars from Israel. It also said North Korea is believed to have built the drones based on Chinese models it obtained in Hong Kong.

Stars and Stripes’ Yoo Kyong Chang contributed to this story.