China is striving for global leadership, and has the economic clout to realize its vision.

The FBI arrested two Chinese nationals over the weekend for illegally photographing a US Navy base in Florida, according to a criminal complaint filed Monday in federal court. It was the second Chinese surveillance operation authorities uncovered in less than two weeks, both on the grounds of Naval Air Station Key West.

Naval Air Station Key West is home to the Joint Interagency Task Force South, which monitors illicit trafficking in the air and at sea. It also houses a sensitive storage facility for weapons and ammunition. In December 2018, authorities stopped a third attempt, by a Chinese student, who was capturing images of buildings and antenna arrays at the same Navy installation.

China has stepped up its espionage activities against the United States in recent years, and constitutes the most severe intelligence threat to the nation, American officials say.

In the latest intrusion, according to court filings, Yuhao Wang and Jielun Zhang drove up to a guard station at Naval Air Station Key West in a blue Hyundai on Jan. 4 at about 8:30 am. The guard asked the men for their military IDs, which they did not have. She told Wang and Zhang to turn around and leave the grounds. Instead, they drove straight onto the property.

The guard, who was not permitted to leave her post, radioed other security personnel with a description of the vehicle. After 30 minutes, Navy security forces finally located the two men. They allowed the security officers to look at their phones and their Nikon camera.

The officers found photographs taken of the property, according to the complaint, including “US military structures on Fleming Key,” a sensitive storage site for weapons and ammunition. Officers also found video Zhang filmed with one of his devices.

Wang and Zhang are charged with entering military, naval or Coast Guard property for the purpose of taking photographs. If convicted, they face up to a year in prison. On Dec. 26, authorities arrested another Chinese national at the same base for the same offense.

The repeated Chinese attempts to access and surveil Naval Air Station Key West represent “Chinese persistence in going after sensitive targets,” Cedric Leighton, a retired US Air Force colonel and intelligence officer, told Quartz in a recent interview.

“They will employ both technical and human intelligence to gain as detailed a picture of US military operations as possible,” he added.

This fall, two Chinese diplomats living in the US were expelled by the government after driving onto a military base in Virginia that is home to a Special Operations unit. The men attempted to evade security forces, only stopping after their path was blocked by fire trucks parked across the road. Authorities reportedly believe one of the men was actually a Chinese intelligence officer posing as a diplomat.

In September, authorities charged a Chinese-born US citizen working as a tour guide in San Francisco with being a Chinese government agent, allegedly passing US secrets to China’s Ministry of State Security.

And as Quartz reported last week, a Chinese national was arrested in Southern California over the summer after attempting to purchase a Harris Falcon III AN/PRC 152A radio, which is designated as a defense article on the United States Munitions List and subject to international arms trafficking regulations. This means the Falcon III, which provides US troops in the field with National Security Agency-certified encrypted communications, cannot leave the country without a special license issued by the State Department.

While the number of arrests may seem high, they’re “just a fraction out of a massive wave of China’s spy operations on US soil,” Jan Neumann, who served as an officer with Russia’s Federal Security Service (FSB) before defecting to the US in 2008, told Quartz.

Juan Michelen and Hector Flores, attorneys for Wang and Zhang, did not immediately respond to a request for comment.