By Liz Kowalczyk

A former surgical resident who had been dismissed from a program in New York City, Cheryl Wang, 42, had no business being at the Brigham. But she blended in with the circulating mass of medical personnel, slipping into restricted areas and suggesting she had connections to an attending doctor.

Once physicians caught on and she was escorted off the property, the Brigham posted her photograph near operating rooms and alerted its sister hospitals in Boston. The next day, she showed up in a conference room at Massachusetts General Hospital for grand rounds — a seminar for medical staff. And when she was told to leave there, Brigham officials followed her straight to Children’s Hospital, where they said she was intercepted and turned away.

It’s unclear what drove Wang’s efforts to infiltrate the hospitals — perhaps a thwarted desire to become a doctor. But Brigham executives said she did not participate in patient care. “Ms. Wang was an observer. She did not touch, treat, or provide care to a single patient,’’ spokeswoman Erin McDonough said. Efforts to reach Wang for this story were unsuccessful.

Yet, her ability to enter restricted operating room suites — without an identification badge — shows how difficult it can be to enforce security in institutions that teem with thousands of patients, families, and staff each day. Brigham officials said their review of security video and other evidence shows she gained access to five operating rooms over two days.

The incident, which became a hot topic among some Brigham operating room staff, also highlights a vexing security problem called “tailgating.

As is the practice at many hospitals, Brigham operating room staff hold their identification badges in front of an electronic card reader to gain access to surgery suites. According to video surveillance and staff accounts, the woman tagged along behind employees during shift changes, slipping in as groups of operating room staff held the door for one another.

During its investigation of Wang, the Brigham discovered that she had been dismissed from the surgical residency program at Mount Sinai St. Luke’s Hospital in New York City, and she had been reported to New York’s state disciplinary board. They said she had forged the recommendation letters that helped her win permission to shadow a Brigham surgeon for two days in September. She returned several months later to wander into operating rooms in scrubs she could have obtained during her September visit, stamped with the Brigham logo.

As a result of the lapse, the hospital said it has strengthened its policy for allowing observers into its 47 operating rooms. Physicians sponsoring a visitor are now required to verify with a student’s educational institution that the student “is in good standing’’ — a safety step that was not taken in Wang’s case. The hospital said it also plans to educate staff about the dangers of tailgating.

“This individual looked and acted like she belonged in our institution. She was wearing our scrubs, knew her way around, understood the hospital culture and terminology, and was familiar with people’s names,’’ McDonough said in a written statement. “Because of this, we let our guard down. We know that in addition to best practice security measures, the safety and security of our hospital requires the vigilance of everyone who works here. All involved are fully committed to providing a secure [operating room] for our patients and staff.”

Tailgating during shift changes is a common problem in hospitals, said Martin Green, president of the International Association for Healthcare Security & Safety and head of security at Baycrest Health Sciences in Toronto. “It’s very difficult to prevent.’’Because people naturally want to hold the door for others, “we always have to teach people to go against the way they were brought up,’’ said Bonnie Michelman, executive director of security at Mass. General.

Electronic card access and security cameras outside operating rooms — both in place at the Brigham — are considered “best practice,” Green said. But given the growing concern about hospital security, other safeguards should be considered, he said. Options include installing subway-style turnstiles — though hospitals are reluctant to do so because they take up a lot of space — or stationing a receptionist or security officer outside operating rooms, which is costly. There are also finger or eye scanner systems — technology that Mass. General tested but did not adopt.

“You need to treat security like an onion,” Green said. “When you get to the inner shells, it becomes more difficult to get there.’’

McDonough said Wang had applied to transfer to the Brigham residency program from Mount Sinai in September after she had been dismissed from that program — a detail she failed to note. As part of her application through the National Resident Matching Program, she attached three recommendation letters. She then asked to shadow a surgeon while her application was pending. He never thought of calling Mount Sinai because he had been introduced to her via an e-mail from another Brigham doctor, McDonough said. She shadowed him on Sept. 20 and 21, 2016. The Brigham declined to identify the doctors involved.

Months later, during the week of Dec. 5, Brigham staff noticed Wang in operating rooms again. She stood on a stool, as observers often do, intently following the surgeons’ work. Two nurses said they believe they saw Wang in operating rooms or locker rooms during other weeks, but McDonough said security officials extensively reviewed video surveillance footage and operating room logs and believe she attended surgeries just two days.

Nurses said Wang also attended patient rounds in thoracic surgery, where cases are discussed. Doctors realized she was not authorized to be there and one confronted her on Dec. 7, the hospital said. She was escorted off the property, and security e-mailed an alert to operating room staff, posted Wang’s photo, and notified Children’s Hospital, which is physically connected to the Brigham, and Mass. General, which is part of the same organization.

Mount Sinai spokeswoman Lucia Lee confirmed that Wang had been dismissed from the residency program last May and said the hospital reported the action to the Office of Professional Medical Conduct in New York. That agency would not comment on what she was reported for or whether it is investigating Wang.

Mount Sinai officials said Friday that a routine background check before Wang was accepted into the residency program raised no red flags, and that she had previously completed a residency program in radiation oncology at a Massachusetts hospital. But during her first year in the Mount Sinai surgery program, Wang had a verbal altercation with a colleague and declined to enroll in a counseling program, so she was dismissed.

The Brigham said Mount Sinai confirmed that the recommendation letters — which are usually sent directly by the authors — were not genuine.

After Wang tried to attend a medical staff discussion in a Mass. General conference room on Dec. 8, investigators from Mass. General and the Brigham interviewed her. Michelman described her as articulate and intelligent. “She said she liked to hear what is going on and be educated,’’ she said.

The hospitals threatened to arrest her if she returned but did not do so then in part because investigators “did not find anything in her background that would cause us to be extraordinarily concerned,’’ Michelman said. But after she left Mass. General, Brigham officers followed Wang as she went directly to Children’s Hospital; they notified the Children’s security team.

Nurses have complained about security at the Brigham in the past. The issue flared two years ago, when Dr. Michael Davidson was fatally shot by a man who was distraught over the death of his 78-year-old mother, who was a patient. At the time, nurses said it was too easy to access patient floors and that they did not have enough input on security decisions.

An agreement reached with the nurses union last June requires the hospital to provide annual safety training to staff, including personal safety and threat assessment, outlines a process for nurses to communicate safety concerns to hospital leaders, and calls for the Brigham to post signs at all hospital entrances that weapons are prohibited and video surveillance is in effect.