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On a Wednesday afternoon in mid-December, a Chinese woman entered the grounds of President Donald Trump's private Mar-a-Lago club in Florida through a service entrance and snapped photos on her cellphone. "Who is Mar-a-Lago?" she said in court following her arrest.

Eight days later, a Chinese student walked around a perimeter fence at a U.S. naval base in Key West, taking pictures of government buildings. Stopped by police, he said he was trying to capture images of the sunrise.

And nine days after that, two more Chinese students drove past a guard at the same naval base. When stopped by security 30 minutes later, they voluntarily displayed the videos and photos they had taken of the base.

Were the incidents isolated cases of tourists mistakenly taking photos in sensitive locations? Or could some or all of the people be part of a spy operation run out of Beijing?

Federal authorities are working to answer those questions. FBI counterintelligence agents are investigating whether the spate of incidents might be part of a coordinated espionage effort, according to a U.S. official and another person familiar with the matter.

In total, four Chinese men have been arrested for trespassing and taking photos at Naval Air Station Key West since September 2018, and two Chinese women have been arrested for trespassing at Mar-a-Lago since March 2019.

"Coincidences take a lot of planning," said Frank Figliuzzi, a former FBI assistant director for counterintelligence.

"These types of brazen and repeated attempts to breach security at two separate facilities in the same region are highly unusual and worthy of serious inquiry by our intelligence community," said Figliuzzi, who is an NBC News analyst.

Figliuzzi said the cases could represent a range of scenarios.

"These attempts could be a distraction from some greater targeting or an attempt to tie up federal security resources in an area responsible for securing the president's residence," he said. "This could also be an attempt to test the Trump administration's ability to react to targeting by one of our greatest adversaries."

Some China experts said they found the incidents at the military installation to be particularly suspicious.

"You don't just saunter onto a base like that, and individual Chinese students are extremely risk averse to this kind of thing," said Robert Daly, director of the Kissinger Institute on China and the United States at the Woodrow Wilson Center.

"All of these people are in the same location, are Chinese, have cameras and backgrounds that are not those of casual tourists," Daly added. "I never thought this could be anything other than a coordinated effort."

Truman Harbor at Naval Air Station Key West, Florida, on Aug. 3, 2019. Danette Baso Silvers / U.S. Navy file

A spokesperson for the FBI in Miami declined to comment, citing the ongoing cases. The Chinese Embassy did not respond to a request for comment.

The first Chinese student arrested at the naval base in Key West was Zhao Qianli, 20, who was taken into custody on Sept. 26, 2018.

Zhao entered the base by walking along the facility's secure fence line and trudging through the beach, court documents say.

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Zhao headed directly to the Joint Interagency Task Force South property, according to court records, where he took several photographs on his Motorola cellphone and his Canon EOS digital camera.

His devices contained photos and videos of sensitive equipment at the facility's "antenna farm," as well as images of warning signs that read "Military Installation" and "Restricted Area," according to court documents.

Zhao initially told military police that he was "lost" and that he was a "dishwasher from New Jersey." In later conversations with the FBI, Zhao said he traveled to Key West to "see the sights, such as the Hemingway House," but there were no images of tourist attractions on his phone, according to his sentencing memo.

Zhao admitted to receiving military training as a university student in China and was found to have a "police blouse" and a People's Republic of China Interior Ministry belt buckle at his hotel, the memo says.

He pleaded guilty to one count of photographing defense installations. U.S. District Judge K. Michael Moore sentenced him to one year in prison.

Zhao's attorney, Hongwei Shang, did not respond to a request for comment. At his sentencing, Zhao's lawyer argued that his actions were motivated by stupidity, not spycraft.

"He's not a spy," she said, according to The Miami Herald, adding: "He committed a stupid mistake. He confessed to it. He just wants to go home."

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The next incident came on March 30, 2019, when Yujing Zhang, 32, tried to enter Mar-a-Lago carrying a trove of electronics, including four cellphones, a laptop computer, a hard drive and a thumb drive.

Zhang said she bought a ticket for an event at the club that had originally been scheduled that Friday night at Mar-a-Lago. There was an event slated for that night, but it had been canceled in the wake of scrutiny of another Chinese woman, Cindy Yang, who built a business around selling access to Mar-a-Lago to wealthy Chinese. In an interview with NBC News, Yang said she had wanted to portray herself as close to Trump with her social network but has no connection to the Chinese government.

Trump was reported to have been golfing near Mar-a-Lago at the time of Zhang's arrest. Law enforcement searched her hotel room and was reported to have found nine USB drives, five SIM cards, another cellphone, a radio frequency device to detect hidden cameras and over $8,000 in cash.

Zhang pleaded not guilty and acted as her own attorney at trial. She was convicted of trespassing.

Zhang was slated to be deported, but her whereabouts are unknown. Spokespersons for Immigration and Customs Enforcement did not return a request for comment. Zhang's initial public defender, Robert Adler, also did not respond to a request for comment.

Lu Jing, 56, was arrested near Mar-a-Lago on Dec. 18. Trump was not at the club at the time, but he was scheduled to be in Florida for the coming holidays.

Jing was initially turned away from the club when she showed up, but she then walked back through a service entrance and began snapping photos on Mar-a-Lago property.

Jing Lu, a Chinese national, trespassed at President Donald Trump's Mar-a-Lago club and was arrested when she refused to leave on Dec. 18, 2019. Palm Beach County Sheriff's Office via AP

A surveillance camera recorded her on the premises, but Jing made it all the way to a high-end shopping district about 1.5 miles away before she was stopped by police.

According to a police report, she raised her arms and shouted "No, no, no," when an officer approached her on Worth Avenue in Palm Beach after Mar-a-Lago staff alerted authorities. Police soon discovered that she was in the U.S. on an expired visa.

When the judge ordered her during a court hearing to stay away from Mar-a-Lago, she asked through a Mandarin translator, "Who is Mar-a-Lago?" She later testified that she paid $200 for a Chinese guide to drop her off at several locations and "made a mistake" by entering the president's private property.

She was found guilty of resisting arrest but was acquitted on a trespassing charge. A judge sentenced her to six months in jail.

"You don't start deleting photos and resisting arrest if you're just a tourist visiting Palm Beach," Palm Beach County State Attorney Dave Aronberg told NBC News.

Jing's public defender did not respond to multiple requests for comment.

Lyuyou Liao, 27, a Chinese national who was in the U.S. on a student visa, was arrested the day after Christmas last year after ignoring warning signs at the Key West naval base and taking photos on his cellphone of the Truman Annex.

Federal prosecutors said Liao entered the base's restricted area by walking around the perimeter fence and following the rocks along the waterline — nearly the identical route taken by Zhao, the earlier trespasser.

When Liao was stopped by military police, he said in broken English that he was "trying to take photos of the sunrise," according to a federal complaint.

On Friday, Liao pleaded guilty to one count of photographing or sketching defense installations. He faces up to one year in prison. Liao's attorney, Daniel Rashbaum, declined to comment on the case.

The most recent incident took place on Jan. 4 at the same naval facility.

Yuhao Wang and Jielun Zhang, both 24-year-old students at the University of Michigan, were charged with entering military, naval or Coast Guard property for the purpose of photographing defense installations.

According to a federal complaint, they drove onto the restricted area in a blue Hyundai sedan after ignoring a guard's instructions to turn around after neither could provide military identification. They were located by security 30 minutes later and found to have taken photos on a Nikon camera and captured videos on their cellphones, the complaint says.

The pair have pleaded not guilty. Their lawyers did not respond to requests for comment.

Aronberg, the Palm Beach state attorney, said he's been alarmed by the number of cases of Chinese nationals' entering sensitive locations in south Florida.

"The facts of these cases are bigger than Mar-a-Lago or any one individual," Aronberg said. "This is about an overall pattern of activity that raises questions and concerns involving national security."