PALM BEACH, FLA. - As palm trees swayed in the ocean breeze, Yujing Zhang approached Secret Service agents in the Mar-a-Lago parking lot.

She said she was going to the swimming pool at the Palm Beach presidential estate and presented agents with two Chinese passports in her name. That raised suspicions with her screeners, but a call to the front desk at Mar-a-Lago revealed a club member with a similar last name and with that, and a possible language barrier, she was waved through.

Mar-a-Lago Arrest Spotlights Security Risks at Trump Estate

Not long afterward, Zhang was arrested carrying four cellphones, a laptop computer, an external hard drive and a thumb drive containing computer malware in an incident that is shining a spotlight on the unique difficulty of fortifying the oceanside Florida estate of President Donald Trump — who was staying at the club that weekend but golfing elsewhere at the time.

Mar-a-Lago Arrest Spotlights Security Risks at Trump Estate

Zhang's arrest has revived concerns about security — particularly cybersecurity — at a presidential refuge that mixes social functions, world diplomacy and extraordinary access to the president. Hundreds of members frequent Mar-a-Lago and the president's other private clubs, which function as working resorts even when Trump himself visits, creating a series of challenges that test the Secret Service.

Mar-a-Lago Arrest Spotlights Security Risks at Trump Estate

Federal officials were looking into whether Zhang was part of a larger effort to gain access to the president and do potential harm, and were combing through her devices and treating the case as a "credible threat,'' according to a U.S. official who was not authorized to speak about the probe and discussed the matter on condition of anonymity.

Democratic lawmakers were inquiring about a possible connection to Li Yang, the Republican donor and spa owner who promised Chinese business leaders that her consulting firm could get them access to the president at Mar-a-Lago.

Democrats on Wednesday called for an investigation into security at Mar-a-Lago, and whether classified information stored there is at risk from hostile foreign governments. House Oversight Committee Chairman Elijah Cummings, D-Md., said he would get a briefing Thursday from the Secret Service.

"We want to make sure that the Secret Service is being the very best that they can be, and we want to find out more about exactly what kind of security they had down there in Florida,'' Cummings said. "I think it's very, very, very, very important that the president be protected. And I feel very strongly about that.''

Trump on Wednesday dismissed the incident, saying it was "just a fluke situation.''

"We have very good control,'' he told reporters.

With the Atlantic Ocean to the east and Florida's Intracoastal Waterway to the west, Mar-a-Lago sits on the Palm Beach barrier island, a 128-room, 62,500-square foot symbol of opulence and power. Long a Trump favorite since he purchased it from the foundation of the late socialite Marjorie Merriweather Post in 1985, the president travels to the estate every few weekends during its winter high season, abandoning Washington's chill for Florida sunshine.

"For the president, I think Mar-a-Lago is not so much a club, but his Xanadu,'' said Chris Ruddy, publisher of Newsmax and a longtime club member and Trump friend. "My feeling is he also sees it as place of destiny and fate because Mrs. Post wanted it as the winter White House.''

While there, Trump has been known to crash weddings, pop in on charity events and, one time, order airstrikes.

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He has not been shy about conducting government business there. It was while hosting Chinese President Xi Jinping that Trump, over a chocolate cake dessert, authorized a missile launch at Syrian airfields after a chemical attack. On another occasion, he and Japanese Prime Minster Shinzo Abe sat on an outdoor patio, as guests dined nearby, and reviewed options for responding to a North Korean ballistic missile test.

Such interactions could make Mar-a-Lago a tempting target, particularly for cyberattacks.

"Mar-a-Lago has not been sufficiently well-defended against not just physical attacks, but against counterintelligence exploits, including digital attacks,'' said David Kris, an authority on foreign intelligence at Culper Partners, a consulting company in Seattle.

Federal agencies spent about $3.4 million per Trump visit, much of it on security, according to an analysis done by the U.S. Government Accountability Office of four 2017 trips. The Secret Service doesn't decide who is invited or welcome at the resort; that responsibility belongs to the club. Agents do screen guests outside the perimeter before they're screened again inside.