By Steve Holland and Karen Freifeld

WASHINGTON (Reuters) - The seizure of classified U.S. government documents from Donald Trump's sprawling Mar-a-Lago retreat spotlights the ongoing national security concerns presented by the former president, and the home he dubbed the Winter White House, some security experts say.

Trump is under federal investigation for possible violations of the Espionage Act, which makes it unlawful to spy for another country or mishandle U.S. defense information, including sharing it with people not authorized to receive it, a search warrant shows.

As president, Trump sometimes shared information, regardless of its sensitivity. Early in his presidency, he spontaneously gave highly classified information to Russia’s foreign minister about a planned Islamic State operation while he was in the Oval Office, U.S. officials said at the time.

But it was at Mar-a-Lago, where well-heeled members and people attended weddings and fundraising dinners frolic on a breezy ocean patio, that U.S. intelligence seemed especially at risk. While Secret Service provided physical security for the venue while Trump was president and afterward, they are not responsible for vetting guests or members.

The Justice Department’s search warrant raises concerns about national security, said former DOJ official Mary McCord.

“Clearly they thought it was very serious to get these materials back into secured space,” McCord said. "Even just retention of highly classified documents in improper storage - particularly given Mar-a-Lago, the foreign visitors there and others who might have connections with foreign governments and foreign agents - creates a significant national security threat."

Trump, in a statement on his social media platform, said the records were "all declassified" and placed in "secure storage."

McCord said, however, she saw no "plausible argument that he had made a conscious decision about each one of these to declassify them before he left.” After leaving office, she said, he did not have the power to declassify information.

Monday's seizure by FBI agents of multiple sets of documents and dozens of boxes, including information about U.S. defense and a reference to the "French President," poses a frightening scenario for intelligence professionals.

"It's a nightmarish environment for a careful handling of highly classified information," said a former U.S. intelligence officer. "It's just a nightmare."

The DOJ hasn't provided specific information about how or where the documents and photos had been stored, but the club's general vulnerabilities have been well documented.

In a high profile example, Trump huddled in 2017 with Japan's then-Prime Minister Shinzo Abe at an outdoor dinner table while guests hovered nearby, listening and taking photos that they later posted on Twitter.

The dinner was disrupted by a North Korean missile test, and guests listened as Trump and Abe figured out what to say in response. After issuing a statement, Trump dropped by a wedding party at the club.

"What we saw was Trump be so lax in security that he was having a sensitive meeting regarding a potential war topic where non-U.S. government personnel could observe and photograph," said Mark Zaid, a lawyer who specializes in national security cases. "It would have been easy for someone to also have had a device that heard and recorded what Trump was saying as well."

White House aides did set up a secure room at Mar-a-Lago for sensitive discussions. That was where Trump decided to launch airstrikes against Syria for the use of chemical weapons in April 2017.

The decision made, Trump repaired to dinner with visiting Chinese President Xi Jinping. Over a dessert of chocolate cake, Trump informed Xi about the airstrikes.

In 2019, a Chinese woman who passed security checkpoints at the club carrying a thumb drive coded with “malicious” software was arrested for entering a restricted property and making false statements to officials, authorities said at the time.

Then-White House chief of staff John Kelly launched an effort to try to limit who had access to Trump at Mar-a-Lago, but the effort fizzled when Trump refused to cooperate, aides said at the time.

(Reporting By Steve Holland and Karen Freifeld; Editing by Heather Timmons and William Mallard)