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A woman used counterfeit credentials and impersonated a congressional spouse to attend a Republican retreat this week on the same day President Donald Trump, Vice President Mike Pence and British Prime Minister Theresa May met with lawmakers in Philadelphia.

The remarkable security breach was announced by Mark Strand, the president of the Congressional Institute, the nonprofit group that hosted the annual closed gathering. He sent a letter to Republicans on Saturday night alerting them after audio of a panel discussing Obamacare was leaked to The Washington Post and other media outlets.

Most of the policy sessions during the event were attended by members, but spouses were invited to the the lunch with Trump and meeting with Pence, with some posting pictures on social media.

It’s unclear whether the female intruder attended those meetings.

Strand’s letter stresses that Capitol Police doesn’t have the identity of the intruder, but says she went through the same security checkpoints that members and aides did to gain access to the meeting.

CNN reached out the Capitol Police for comment, but has not received a response yet. It’s unclear why the intruder was removed from the event, but law enforcement does not know who she was or how she got it. Strand apologized in his letter and vowed to out new security protocols in place in the future.

“The Congressional Institute is continuing to investigate this breach in order to fully understand how it happened and to ensure it does not happen again,” Strand says in the letter, according to a copy CNN obtained from a congressional source.

Strand said the woman “infiltrated” the secure gathering that took place at the Loews Hotel across from City Hall between 7:30 a.m. ET and 6:30 p.m., when she was escorted out. He said she “misrepresented herself on multiple occasions” as a spouse of a member of Congress. Most of the policy sessions during the event were attended by members, but spouses were invited to the the lunch with Trump and meeting with Pence, with some posting pictures on social media.

The recording of the private session with members discussing plans to dismantle the health care law captured many GOP members expressing concerns about the fallout that could disrupt the market and potentially cause the constituents to lose coverage. Other details of sessions with Pence were also distributed from what was described as “an anonymous source.”

Rep. Cathy McMorris Rodgers, R-Washington, the chair of the House GOP conference that hosts the event, told CNN in a statement, “leaks from internal member discussions are unacceptable.”

She added that the Congressional Institute is working with Capitol Police and the 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.”

The Loews Hotel where the retreat took place was essentially locked down during the event that began Wednesday and wrapped up Friday morning.

Members, their family members and aides had special credentials to enter the floors where the closed meetings took place.

Security was especially tight on Thursday, the day that Trump, Pence and May arrived from Washington. Barriers encircled the hotel and the streets around the downtown area were closed down.

Protesters gathered but were kept away from the immediate vicinity of the hotel. Reporters covering the retreat were only allowed in a large ballroom set up as a press center across the street at the Marriott.

Members of Capitol Police were posted at the room, enforcing rules that prevented the press to roam around. A small press pool were escorted by security to attend the speeches by Trump, Pence and May, but other than those few reporters, none gained access to the floors where members were planning next steps on tax reform, Obamacare repeal and other agenda items.