In a shocking incident that happened earlier this month, United States military commanders say a small group of armed Mexican soldiers briefly detained a pair of U.S. soldiers who were conducting a routine surveillance mission.

What's worse: The U.S. soldiers were sitting on the Texas-side of the border when the incident occurred.

What are the details?

United States Northern Command told CNN the incident happened in the afternoon on April 13.

"On April 13, 2019, at approximately 2 p.m. CDT, five to six Mexican military personnel questioned two U.S. Army soldiers who were conducting border support operations in an unmarked [Customs and Border Protection] vehicle near the southwest border in the vicinity of Clint, Texas," the military command said.

During the incident, the Mexican soldiers, who were armed with high-powered assault rifles, ordered the U.S. soldiers — one sergeant and a private — out of the unmarked vehicle, pointed their firearms at the U.S. personnel, and briefly detained them.

The Army sergeant, who was armed with his service pistol, was even disarmed, an incident report said.

"After a brief discussion between the soldiers from the two nations, the Mexican military members departed the area. The U.S. soldiers immediately contacted CBP, who responded quickly. Throughout the incident, the U.S. soldiers followed all established procedures and protocols," John Cornelio, deputy director of public affairs for NORAD and U.S. Northern Command, told Newsweek.

The Mexican military personnel moved "tactically fast," preventing the U.S. soldiers from radioing for backup. To make matters worse, both the uniforms of the Mexican soldiers and their truck were not marked with "identifiable seals or symbols."

The incident took place south of the border fence, but north of the Rio Grande, the official border demarcation at that section of the border.

After the incident, Customs and Border Protection agents arrived at the scene, along with members from the soldiers' regiment.

What sparked the incident?

According to U.S. Northern Command: "An inquiry by CBP and DOD revealed that the Mexican military members believed that the US Army soldiers were south of the border."

"Though they were south of the border fence, U.S. soldiers remained in U.S. territory, north of the actual border," the statement explained.