Officials who organized this week's closed-door conference of House and Senate Republicans believe an "unauthorized person" gained access to the event and may be the source of audio recordings distributed to the press.

In a letter to lawmakers obtained by POLITICO, officials from the Congressional Institute said that a woman "infiltrated" the GOP retreat -- held in downtown Philadelphia -- on Thursday and identified herself as a lawmaker's spouse. She snuck into the event on the same day that President Donald Trump and Vice President Mike Pence addressed lawmakers.

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"She was able to gain access using counterfeit credentials," Institute officials wrote to lawmakers, who noted that she went through security screening and was ultimately asked to leave at 6:30 p.m. that night.

They added that they are working with Capitol Police to identify the intruder and pledged to keep members of Congress in the loop about the investigation.

"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 including more sophisticated RFID badges with the ability to identify in a more secure way," Institute officials wrote.

The presence of an unauthorized guest at the retreat is a clue to a possible source of audio recordings obtained by POLITICO, the Washington Post and other news organizations. The recordings featured private discussions by lawmakers about health care and other policy topics.

In a statement, House Republican Conference Chairwoman Cathy McMorris Rodgers (Wash.) - whose role in the GOP hierarchy is to schedule such events - welcomes a probe into who made and leaked the recordings.

"Leaks from internal members discussions are unacceptable," McMorris Rodgers said. "I'm pleased to learn the Congressional Institute, who organizes the annual retreat, will work closely with the Capitol Police and Sergeant at Arms to get to the bottom of this serious breach, as these conversations are intended to allow members to candidly discuss how to address the issues facing the American people."