Two Chinese nationals who attend the University of Michigan were arrested in Key West, Florida, over the weekend after they were caught taking pictures of a naval base.

The duo, Yuhao Wang and Jielun Zhang, were arrested after entering Sigsbee Annex in the Naval Air Station. Guards had denied the 24-year-old men entry to the military facility, but the two entered anyway. Authorities eventually caught the students after 30 minutes of searching. They were found with cellphones and a Nikon camera, according to the Florida Keys Keynoter.

Once caught, both students acknowledged they disregarded guard instructions to not enter the facility.

An FBI affidavit said, "U.S. Navy Security Forces obtained consent to look at the devices and observed photographs taken on the Sigsbee Annex property, including U.S. military structures on Fleming Key."

The students were charged for "entering a U.S. Naval property for the purpose of photographing defense installations," the Keynoter reported. They both appeared in federal court in Key West on Monday. Their detention hearing is scheduled for Friday.

Wang and Zhang are the third and fourth Chinese nationals to be arrested for taking pictures of military installations at the Key West naval station.

One day after Christmas, 27-year-old Chinese student Lyuyou Liao was arrested for taking pictures at the Truman Annex, while 20-year-old Zhao Qianli was arrested for the same offense in September 2018. Qianli pleaded guilty last February to one count of photographing defense installations. Meanwhile, Liao is still waiting to be arraigned, the Miami Herald reported.

The incident comes as Chinese espionage efforts appear to be on the rise.

As TheBlaze reported, two Chinese officials were "secretly expelled" from the U.S. after they breached a "sensitive" military installation last September in Virginia. It was the first time in more than 30 years that the U.S. had expelled Chinese officials from the country.