Fort Worth police said a search for an armed man inside the John Peter Smith Health Center South Campus in Fort Worth turned up nothing.
Police set up a perimeter around the clinic after an employee said she thought she saw a man wearing a black sweatshirt with a long gun enter the clinic at about 7:45 a.m. The man was believed to be barricaded inside the building.
Lt. Paul Henderson said employees inside the building locked down and sheltered in various offices. Police said approximately 12 people were in the building at the time of the call.
Employees asked patient Jesse Decker to evacuate when he arrived for his early-morning appointment. He said he heard a woman yelling that a man inside the clinic had a rifle while he was signing in.
"It's real scary," patient Jesse Decker said. "I mean, you think there's a guy with a gun, and you don't know where he (is) at."
Police officers searched the building and were unable to locate a gunman or a gun, but said they took precautions because they believed the employee's report was credible.
Officers later released employees who had been locked down inside the building. Many were brought out of the building in handcuffs before their identities were verified.
"Whenever you have a potential hostage situation or a barricaded person, when SWAT makes entry into that type of a situation, the protocol is that you basically handcuff anyone that you come in contact with, especially if they're anywhere near the description of the suspects," Henderson said. "So they were placed in handcuffs for their safety and also for ours until we could finish clearing that part of the building.”
There were no reports of shots fired and no injuries.
The clinic, north of Interstate 20 and east of Interstate 35W in the 2500 block of Circle Drive, was closed for the remainder of the day.
The clinic sees up to 150 patients per day and contacted patients who had appointments to reschedule.
NBC DFW's Amanda Fitzpatrick contributed to this report.