(Reuters) - Two people died when an incoming cargo plane carrying UPS packages swerved off the runway at an airport in Charleston, West Virginia, on Friday and went over a hillside, officials said.

The two were the only people on board the aircraft, which crashed at Yeager Airport in Charleston, West Virginia, said Bill Carpenter, spokesman for Kanawha County Emergency Medical Services.

It was not immediately clear what caused the accident.

The small feeder aircraft, inbound from Louisville, Kentucky, was carrying United Parcel Service Inc packages but was not owned by the company, according to Jim Mayer, a UPS spokesman.

“We do not have additional information to share about the carrier, or condition of the aircraft or crew,” Mayer said in an email on Friday. It was not known if the victims were UPS employees.

The U.S. National Transportation Safety Board said Friday it was sending an investigative team to the crash site.

The airport said it would remain closed until at least Saturday to general and commercial aviation.

After the crash, reports of a diesel fuel leak were submitted to a local water spill hot line because of concerns that aircraft fuel could reach a tributary of the Elk River, the city’s water treatment service provider, West Virginia American Water, said in a Facebook post.

Staff was collecting water samples near the crash site, the company said, adding that its treatment plant has not detected any changes in water quality.