WASHINGTON (THE WASHINGTON POST) - To get inside Mar-a-Lago, where President Donald Trump was staying last Thanksgiving, 18-year-old Mark Lindblom walked along the beach until he reached an underground tunnel, leading straight to the luxury club.

The narrow corridor dipped beneath South Ocean Boulevard, a shortcut so Mar-a-Lago Club members didn't have to cross the street on their way to and from the beach.

There was a sign posted right outside the tunnel, warning, "UNITED STATES SECRET SERVICE RESTRICTED AREA", according to federal court documents. But Lindblom, who was not a member of the club, was undeterred.

"I wanted to see how far I could get," the college freshman told a federal magistrate on Tuesday (May 28), the Palm Beach Post reported.

As it turns out, he got pretty far: past Secret Service and all the way inside.

For sneaking onto the property, Lindblom pleaded guilty on Tuesday to entering or remaining in a restricted building where the President was visiting during the Thanksgiving holiday.

Lindblom, reportedly a business student at the University of Wisconsin, was sentenced to a year of probation and US$25 (S$34.50) fine after prosecutors acknowledged he had no ill intent, that he intended only to take pictures on his cellphone.

"We have no reason to believe he had a political, criminal or terroristic purpose," assistant US attorney John McMillan told the court, according to the Palm Beach Post. "It was a foolish decision he did on a lark."

The security breach is the latest incident at Mr Trump's private "Winter White House" in recent months, provoking concerns that inadequate security measures at the club could leave it vulnerable to more serious threats.

In Lindblom's case, his defence attorney, Mr Marcos Beaton Jr, told the court his client had an easy time getting through at least the first layer of Secret Service agents, as the Palm Beach Post reported.

The Washington native was spending the holiday with family at nearby beachfront property, though his family also were not members of Mar-a-Lago.

Upon entering the tunnel on Nov 23, Lindblom squeezed in line with the other club members awaiting a security check by the Secret Service.

Once he reached the checkpoint, he surpassed a screening for weapons or other dangerous items, the Secret Service said in a statement to The Washington Post. Then, he simply "walked on through", Mr Beaton said - lingering on the grounds for 20 minutes before the Secret Service found him near the club pool, the Palm Beach Post reported.

The Secret Service said in its statement that Lindblom did not come into contact with the President or First Lady "because of the layered security system in place at the club". He was arrested after appearing out of place among other club members, the Secret Service said.

"(H)is actions appeared inconsistent with that of a member or a guest," the service said. "At that time, Secret Service personnel approached him and he was detained without further incident."

Security concerns at Mar-a-Lago accelerated last month, when a Chinese national carrying a thumb drive containing malicious software was arrested after gaining access to Mar-a-Lago despite not being a member.

The woman, Zhang Yujing, approached the first security checkpoint and told officials she was there to go swimming, according to the criminal complaint. She raised some concern when asked if she was a relative of another "Zhang" on the member list - but the security official thought her hesitation was because of a language barrier and decided to let her through.

She was not stopped until a receptionist grew suspicious, asking why Zhang was there. Zhang claimed she was there for an event that did not exist, and the receptionist promptly called Secret Service.

A search of her belongings turned up no swimsuit, but rather four cellphones, a laptop, a hard drive and a thumb drive containing "malicious software", plus other suspicious electronics equipment in her hotel room.

The Secret Service said in a statement at the time that the agency "does not determine who is invited or welcome at Mar-a-Lago; this is the responsibility of the host entity", referring to Mar-a-Lago management.

Zhang's arrest on charges of lying to Secret Service agents and entering a restricted building alarmed Democratic congressional leadership, who promptly asked FBI director Christopher A. Wray to launch a review of Mar-a-Lago's security.

As their April letter noted, the President sometimes handles classified information while staying at the club, and sometimes invites heads of state to meet there. In February 2017, for example, Mr Trump was blasted by Democrats after reviewing national security matters with Japanese Prime Minister Shinzo Abe at a dinner table on the Mar-a-Lago terrace in full view of casual diners.

"These potential vulnerabilities have serious national security implications," Democratic leadership wrote to Mr Wray in April.

It was the second letter in as many weeks that Democrats wrote to Mr Wray requesting an investigation connected to Mar-a-Lago. In March, a frequent club guest and Republican donor, Li "Cindy" Yang, was accused of selling access to Mr Trump to Chinese nationals who attended events where Mr Trump was present.

The FBI has since opened a public corruption investigation into Yang, examining whether she illegally funnelled money from Chinese nationals into the President's reelection campaign, the Miami Herald reported. Yang has denied any wrongdoing, and her attorneys say she has no personal connection to the President.

On Tuesday, US Magistrate Judge William Matthewman said it appeared "pretty clear" to him that Lindblom's offence was rather a "youthful indiscretion more out of misplaced curiosity", the Palm Beach Post reported from the courtroom.

A remorseful Lindblom said he understood the seriousness of his offence given the high-stakes nature of the Secret Service agents' jobs.

"I'm sorry for wasting their time," he said.