PALM BEACH, Fla. — President Donald Trump relishes the comforts of his Mar-a-Lago estate for repeated weekends away from Washington, but former Secret Service and intelligence officials say the resort is a security nightmare vulnerable to both casual and professional spies.

While Trump’s private club in South Florida has been transformed into a fortress of armed guards, military-grade radar, bomb sniffing dogs and metal-detection checkpoints, there are still notable vulnerabilities, namely the stream of guests who can enter the property without a background check.

And security experts warn that the commander in chief’s frequent visits — four since he took office in January — afford an unprecedented opportunity for eavesdropping and building dossiers on the president’s routines and habits, as well as those of the inner circle around him. They add that with each repeat visit, the security risk escalates.

“The president is the biggest, richest intelligence target in the world, and there is almost no limit to the energy and money an adversary will spend to get at him,” said David Kris, a former Obama-era assistant attorney general for national security.

“Whose responsibility is it to prevent foreign intelligence? That’s a very good question that remains unanswered" — former Secret Service agent

Former Secret Service agents said the setup at Mar-a-Lago and the president’s other regular clubs presents challenges that their agency wasn’t built to deal with. The Service’s main job is to protect the president from physical threats and monitoring for wiretaps and other listening devices — but not from the kinds of counterespionage challenges presented by the president’s choice to eat, sleep and work at a club accessible to anyone who can get a member to invite them in.

“Whose responsibility is it to prevent foreign intelligence? That’s a very good question that remains unanswered,” said one former Secret Service agent. “Is it the FBI? They’re not involved in protection. It’s not the CIA because they can’t spy on U.S. citizens.”

At the White House, visitors must undergo a rigorous background screening before they’re let in the door. Agents scan every visitor’s full name, birth date, Social Security number, city of residence and country of birth.

But at Mar-a-Lago, gaining entry doesn’t require that degree of disclosure. Guests entering the club go through multiple security checkpoints staffed by the Secret Service looking for weapons or other immediate threats. But there’s only one requirement to produce a photo ID, and the club itself does not ask guests to provide their names or other information when they enter through the main wrought-iron gated door.

The club also serves as a venue for ticketed public events. Hosts for the slate of political and charity dinners booked at the president’s part-time home from now to the end of the club’s season in May told POLITICO the only request for information about attendees has come from the club itself. And all they’re asked to provide is a name, not additional information that can be used for Secret Service background checks in the event the president is in residence.

“One could send a source to attend and report on atmospherics, the buzz, attendees, rumors, etc.,” Michael Hayden, the former head of the CIA and the National Security Agency, and former director of national intelligence, wrote in an email.

Spies don’t even need to physically go to Mar-a-Lago to do their work. Lists of the club's nearly 500 exclusive dues-paying members have leaked in recent weeks to the news media, giving foreign intelligence the names of potential targets for surveillance, blackmail or bribes that can help them get closer to the president.

What’s more, a page on the Mar-a-Lago website — accessible to the public with just a little search engine sleuthing — reveals the names, work email addresses and phone numbers for more than a dozen critical club employees, including the managing director, who has special Secret Service clearance to get up close to Trump, the chief of security, the housekeeping director and the official in charge of food and beverage services. All would be obvious targets for operatives trying to get information on the president or others in his entourage.

“Hostile intelligence services would love to plant bugs in a place like this,” said John McLaughlin, a former acting CIA director.

White House spokeswoman Hope Hicks referred questions about Mar-a-Lago’s security to the Secret Service, which cited its own long-standing policy of declining comment on its operations. Mar-a-Lago officials, meantime, referred calls to the Trump Organization, which did not respond to a request for comment.

In February, the president and his staff remained on Mar-a-Lago’s dining terrace while responding to a report of a North Korean missile test. Guests tweeted out photographs of the scene as it unfolded, prompting lawmakers to complain about the apparent security breach.

Republican Rep. Jason Chaffetz, chairman of the House Oversight and Government Reform Committee, sent White House chief of staff Reince Priebus a letter last month asking a series of questions about the North Korea incident, as well as how guests, employees and residents at Mar-a-Lago are vetted “in order to ensure that they are not foreign agents or spies on behalf of a foreign government.”

In a one-paragraph response sent Tuesday, White House legislative affairs director Marc Short told the lawmaker that no classified information or documents were reviewed or discussed during the president’s dinner meeting with Japanese Prime Minister Shinzo Abe.

“Proper security protocols are adhered to at all times at Mar-a-Lago,” Short added, while also promising to provide the lawmaker with a secure classified briefing on the topic.

The Utah Republican in an interview said the Trump administration had “satisfied our concerns,” though his spokeswoman said they are still trying to arrange the briefing. “We’ll keep an eye on it,” Chaffetz said. “I think they know that we’re keenly concerned about it and aware of it and keeping a watchful eye.”

““No one ever ran a residence that served as a hotel. It’s a security nightmare by having such a turnstile of folks coming and going” — Douglas Brinkley, presidential historian

Other presidents have had regular private vacation retreats away from the secure federal compound at Camp David, in western Maryland. The California estates owned by Presidents Richard Nixon and Ronald Reagan were both referred to as the Western White House. President George H.W. Bush vacationed at Kennebunkport, Maine, and President George W. Bush traveled frequently to his ranch in Crawford, Texas.

But all of those were private homes, off-limits except to people invited by the president. The same was true of the rambling Martha’s Vineyard escape used by President Barack Obama and his family.

“No one ever ran a residence that served as a hotel,” said presidential historian Douglas Brinkley. “It’s a security nightmare by having such a turnstile of folks coming and going.”

Guests have been flocking to Mar-a-Lago since Trump’s inauguration — a boom in interest that has helped charity and political events sell out their tickets to guests hoping they might catch a glimpse of the president, or at least a chance to mingle with his aides. Michael Barnett, chairman of the Palm Beach GOP, which has sold more than 680 tickets for a March 24 Lincoln Day Dinner at Mar-a-Lago, said he hasn’t been asked to collect or provide background information on people buying tickets. “I’m sure our guests won’t have any problem giving out that information if that’s what’s required from the Secret Service,” he said.

People at the club have been able to witness the presidency up close. Trump and the Japanese prime minister stopped by a wedding at the club in February soon after their joint news conference on the North Korea situation. Last weekend, Trump and Attorney General Jeff Sessions were spotted by the Palm Beach Post shaking hands with club members and guests just outside a charity event. On Sunday morning, Ivanka Trump posted a picture on her Instagram account of her three young children on the club’s croquet lawn.

But meeting the president and his team isn’t even needed for spies to do their job effectively. Eavesdropping on conversations among Trump and his senior staff can certainly generate useful pieces of information, but security experts said seemingly mundane pieces of data can be just as useful: Like the routine that aides have every morning for going to the gym or eating breakfast, the names of the waiters and other club staffers whom Trump favors and the housekeepers who work on the president’s private suite and the rooms where his aides work.

“What you’re doing is you’re making it easier for foreign intelligence by telegraphing what you’re going to do every single weekend,” said the former Secret Service agent. “If I know there’s a 50-50 chance, I’m going to try to get in there.”

Kyle Cheney contributed to this report.