WASHINGTON (WASHINGTON POST) - For hours, the intruder strolled around what should have been one of the most tightly secured buildings in the country.

Inside the Loews Hotel in downtown Philadelphia at various points last Thursday (Jan 26) were President Donald Trump, Vice-President Mike Pence, House Speaker Paul Ryan, and scores of other Republican members of Congress. Reporters were kept out, and only a few select staff, family members and outsiders were allowed to participate in the private GOP policy retreat.

But at least one unauthorised person made it inside, and while it is unclear to what degree the country's top leaders were in physical danger, their circle of trust was undoubtedly breached.

A person secretly recorded closed sessions on national security and healthcare attended by many dozens of GOP lawmakers who had gathered for a private discussion of some of the thorniest legislative issues of the moment, as well as a question-and-answer session with Mr Pence.

The recordings were anonymously e-mailed that night to reporters for The Washington Post and other news outlets, who published stories exposing major qualms inside the GOP over the party's plans to roll back Democratic healthcare reforms and a looming debate between defence hawks and advocates of fiscal rectitude.

Mr Pence, meanwhile, made news by committing to pursue an investigation into unfounded accusations of widespread voter fraud.

The intruder's identity is not known; the individual communicated with The Washington Post anonymously via e-mail. The Post reported the contents of the recordings after confirming their authenticity with quoted lawmakers or their staff.

Several lawmakers said they were outraged by the infiltration and have demanded answers on how an interloper made it inside Republicans' sanctum sanctorum.

"Members want to be able to have a candid discussion about issues in that setting," said Representative Charlie Dent, a moderate from Pennsylvania. "I have to think most of my colleagues are very upset about how this could have happened."

"If someone can get in and we don't know who it is, they could have gotten in and been a dangerous person," said Representative Dave Brat, a conservative from Virginia. "Just from the security standpoint, that's not good."

On Tuesday, Representative Cathy McMorris Rodgers, Republican-Washington, the chairman of the House Republican Conference, told Republicans in a private meeting on Capitol Hill that an "active criminal investigation" was underway to determine the identity of the intruder.

The president of the Congressional Institute, the private non-profit tied to Republican lobbyists that organises the retreat each year, told lawmakers in an e-mail late Saturday that an "unauthorised person" infiltrated the retreat on Thursday for nearly 11 hours using "counterfeit credentials." The intruder was later ejected.

The woman "misrepresented herself on multiple occasions to retreat organisers as the spouse of an elected official," the president, Mark Strand, wrote."We are working closely with Capitol Police to ascertain the identity of the woman in question," he added. "In the meantime, we have already initiated efforts to develop new security protocols in order to better protect the internal nature of these meetings moving forward."

The e-mail did not indicate whether the woman who was ejected was the person who made the recordings. Mr Strand declined to comment further, citing a "very active police investigation."

Also unclear is whether the intruder or intruders could have posed a physical threat to Mr Trump, Mr Pence or lawmakers.

Mr Strand, in his e-mail to members, said the woman who was ejected "entered the event through the same security checkpoints as every other attendee (i.e. magnetometres, police checkpoints etc.)."

A spokesman for the US Capitol Police, which had primary responsibility for security at the event, did not respond to multiple requests for comment.

US Secret Service (USSS) spokesman Cathy Milhoan said Capitol Police were primarily responsible for the event's security and said the agency's Philadelphia Field Office was "assisting with the matter".

Mr Trump and Mr Pence, she added, were not in physical danger - although at least one of the recordings appears to have been made while Mr Pence was in the room with the unauthorised person.

"The USSS relies on a multi-layered security approach and we are confident in the protection operation for the President and Vice President," Ms Milhoan said.

Should the intruder be identified, he or she could face charges under local trespassing or wiretapping laws.

In Pennsylvania, it is a felony crime to record a conversation unless all of the recorded parties consent - though that applies only if a person speaks under the expectation that his or her remarks will not be recorded "under circumstances justifying such expectation".

Mr Cameron Kline, a spokesman for the Philadelphia district attorney's office, declined to comment, citing an office policy not to confirm, deny or otherwise discuss pending investigations.

Top congressional leaders, meanwhile, have been forced to reassure their colleagues that their private deliberations will remain that way.

Ms McMorris Rodgers said in a statement on Saturday that she was "pleased" the matter was under investigation.

"Leaks from internal member discussions are unacceptable," she said, adding that "these conversations are intended to allow members to candidly discuss how to address the issues facing the American people."