CHICAGO (CBS)– Jennifer Eldridge, a pharmacy technician barricaded herself and others in Mercy Hospital during the shooting.

Four people were shot after a gunman opened fire Monday afternoon. Sources say the gunman first targeted a woman in the parking lot and then entered the hospital.

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“We didn’t see anything, we could only hear the commotion,” Eldridge said.

Eldridge and others were hiding in the pharmacy, right outside of the lobby. They were able to hear the gunman outside of the building.

She heard what she assumes was the gunman having an argument with an unidentified person.

Mercy Hospital had recently gone through an active shooter drill on Nov. 8. Eldridge said they followed protocol by closing shutters and calling for help.

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At one point, she thought the gunman was trying enter the pharmacy. Someone was grabbing the handle of the door, attempting to enter, she explained.

“My boss decided to close his door and we barricaded ourselves inside of there,” she said.

Eventually, the commotion died down and the police escorted Eldridge and the others out of the building. Most left without their belongings.

“It was a sigh of relief,” she said.

The pharmacy technician explained there were patients waiting in the lobby at the time of the incident.

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“You just hope everybody made it out okay,” she said.