CHICAGO (CBS) — When Dr. Tamara O’Neal, the first victim of the deadly shooting at Mercy Hospital and Medical Center, was brought to the emergency room at the University of Chicago on Monday, Dr. John Purakal was not prepared for what happened next.

“I recognized her immediately. It took me a second to kind of get over that shock, and then I tried to help any way possible,” he said.

READ MORE: Governor Pritzker Announces $300 Million In Grants For Daycare Centers

Purakal’s colleagues realized what was happening, and asked him to let them take over the primary treatment. Despite their best efforts, O’Neal could not be saved.

“We tried everything we could, and I really wish we could have done more. I wish we could have prevented it, and seen it coming, but we couldn’t,” Purakal said.

O’Neal had done her medical residency at the University of Chicago, so Purakal and other doctors there knew her well.

Purakal said what he can do is remember the good times they shared during four years of friendship, much of it spend at the very hospital where she died doing what she loved.

“She was practicing at Mercy, which is someplace that we trained, and we did shifts together there, and we saved lives there together, and she just loved it,” he said.

READ MORE: Buffalo Grove Police Shot And Killed Man Who Fired Shots While Armed With 2 Guns

Purakal was working in the trauma center at the University of Chicago when Police Officer Samuel Jimenez also was wheeled into the hospital, surrounded by fellow officers.

“The injuries suffered were too much,” Purakal said.

Jimenez, a 28-year-old husband and father of three, was shot inside the Mercy Hospital lobby, after he ran inside to confront gunman Juan Lopez, after the 32-year-old man had executed his former fiancée in the hospital’s parking lot.

Chicago Fraternal Order of Police President Kevin Graham said, despite the danger inside the hospital, Jimenez did not flinch from his oath to serve and protect.

“He did just that, but he did so with his life,” Graham said.

Lopez also shot and killed 24-year-old pharmaceutical resident Dayna Less as she stepped off an elevator before Lopez himself was killed. Police have said it’s unclear if Lopez took his own life, or was killed by responding officers.

MORE NEWS: Obama’s Back In Chicago To Celebrate The Holiday Season

The Cook County Medical Examiner’s office was expected to perform autopsies for Lopez and the three victims on Tuesday.