The gunman wasn't injured either, although he fired a shot into the ceiling in what Hahn said looked like a suicide attempt.
Shortly after arriving at the clinic, the man placed the gun on the receptionist's counter and told her he wanted a pill to kill himself.
Heather Lord, 21, told the man to wait while she found someone to help him. While Lord went to get Hahn, another staff member called 911 at 5:46 p.m.
Hahn, 37, introduced himself to the man and tried to find out why he was upset.
"I tried to engage him and keep him talking," Hahn said. "He said, 'I want to die right now and I want a pill to do it.'"
Hahn told him he would look for something if the man put the gun down. But the man never took his hands off the weapon, Hahn and Lord said.
All the while, Hahn motioned for the patients and staff to leave the building.
At one point, the phone started ringing. And ringing. Lord didn't want to answer it because it was right beside the man with the gun.
The man started cursing, asking her to answer the phone.
"That was the worst moment because I thought he was going to lose it," Hahn said.
The man picked up the phone himself. They couldn't remember what he said to the person on the other line.
Lord left the building and Hahn was left alone with the man. Hahn kept the man talking as he inched his way toward the door. Hahn left after the shot was fired.
"Once he fired the gun I didn't think I could help anymore," he said.
Staff members waiting outside were horrified when they heard the gunshot.
"That was a sick feeling," Lord said.
They were relieved when Hahn came outside safely, but the drama still wasn't over.
Staff members tried to keep new patients from driving into the parking lot because they didn't know what the man would do next. A neighbor used his car to block the entrance.
Police arrived on the scene and were talking to Hahn when they saw the man in the parking lot.
Hancock Police Officer Mike Ruppenkamp tried talking to the man, who still held the gun. Several Maryland State Police officers surrounded him.
Ruppenkamp said he tried to keep the man's attention focused on him while a trooper tackled him from behind.
The man was taken to Washington County Hospital for a psychiatric evaluation.
He remained under doctor's care Wednesday, said Hancock Police Sgt. Shawn Tasker.
Police won't decide whether to charge him until they get the final psychiatric report, Tasker said.
The health center canceled its remaining appointments Tuesday night, but reopened as usual at 8 a.m. Wednesday.
Hahn and Lord went home early, though.
Lord, who lives in Hedgesville, W.Va., said the only thing she remembers about the man's appearance is that he was wearing a black and green jacket.
"I can't tell you what he looks like, I can just see that gun pointing at me," she said.
Hahn said the incident reminded him that many of the people who come to the center have a lot of anguish and pain in their lives.
Hahn, who lives in Berkeley Springs, W.Va., began working at the health center two months ago after finishing his residency in York, Pa. He came to Hancock as part of the National Health Service Corps, a program that brings doctors to underserved areas.
Despite the frightening experience, Hahn said he loves the area and plans to stay.
Hahn doesn't hold a grudge against the man, but said he won't ever see him as a patient.
"It was a cry for help and I hope he gets it," Hahn said.