Several other people were injured Thursday night when gunfire erupted at the facility near the Indianapolis International Airport, police spokesperson Genae Cook said.

At least four were hospitalized, including one person with critical injuries. Another two people were treated and released at the scene, she said.

The shooter wasn’t immediately identified, and investigators were in the process of conducting interviews and gathering information. Cook said it was too early to tell whether the shooter was an employee at the facility.

“We’re still trying to ascertain the exact reason and cause for this incident,” she said.

It was the latest in a recent string of mass shootings across the U.S. Last month, eight people were fatally shot at massage businesses across the Atlanta area, and 10 died in gunfire at a supermarket in Boulder, Colorado.

It was at least the third mass shooting this year in Indianapolis alone. Five people, including a pregnant woman, were shot and killed in January, and a man was accused of killing three adults and a child before abducting his daughter during at argument at a home in March.

Police were called to reports of gunfire Thursday just after 11 p.m., and officers “came in contact with (an) active shooter incident,” Cook said. The gunman later killed himself.

“The officers responded, they came in and did their job. A lot of them are trying to face this, because this is a sight that no one should have to see,” Cook said.

FedEx released a statement saying it is cooperating with authorities and working to get more information.

“We are aware of the tragic shooting at our FedEx Ground facility near the Indianapolis airport. Safety is our top priority, and our thoughts are with all those who are affected,” the statement said.

Family members gathered at a nearby hotel to await word on loved ones. Some said employees aren’t allowed to have their phones with them while working shifts at the facility, making it difficult to contact them, WTHR-TV reported.

Live video from news outlets at the scene showed crime scene tape in the parking lot outside the facility.

A witness who said he works at the facility told WISH-TV that he saw a man with a gun after hearing several gunshots.

“I saw a man with a submachine gun of some sort, an automatic rifle, and he was firing in the open,” Jeremiah Miller said.

Another man told WTTV that his niece was sitting in the driver’s seat of her car when the gunfire erupted, and she was wounded.

“She got shot on her left arm,” said Parminder Singh. “She’s fine, she’s in the hospital now.”

He said his niece did not know the shooter.