A medical transport plane crashed and caught fire on Tuesday in northern Arizona on the Navajo Nation, resulting in the deaths of four people, according to a statement from the tribe.

The Federal Aviation Administration and CSI Aviation reported that a Beechcraft King Air 300, operated by CSI Aviation, departed from Albuquerque, New Mexico. Onboard were two pilots and two healthcare providers. The plane crashed in the early afternoon near Chinle’s airport, about 300 miles northeast of Phoenix.

“They were trying to land there and unfortunately something went wrong,” district Police Commander Emmett Yazzie said.

The crew intended to pick up a critically ill patient from a federal Indian Health Service hospital in Chinle and return to Albuquerque, explained Sharen Sandoval, director of the Navajo Department of Emergency Management. The patient’s whereabouts and condition were unclear by Tuesday evening.

Reports of smoke at the Chinle airport started coming in at 12:44 p.m., stated Sandoval. The cause of the crash remains unknown, according to the tribe. An investigation is being conducted by the National Transportation Safety Board and the FAA.

CSI Aviation conveyed their “great sadness” in a statement confirming the deaths and expressed condolences to the victims’ families and loved ones. The names of those who died have not been released.

The company is cooperating with the investigation, according to the statement.

Navajo Nation President Buu Nygren said in a social media post that he was heartbroken to learn of the crash.

“These were people who dedicated their lives to saving others, and their loss is felt deeply across the Navajo Nation,” he said.

Due to limited hospital facilities offering advanced or trauma care, medical air transports from the Navajo Nation are quite common. The Chinle airport is one of several tribal-owned and managed airports on the reservation, which covers 27,000 square miles across Arizona, New Mexico, and Utah, making it the largest reservation of any Native American tribe.

In January, a medical transport plane crashed in Philadelphia, killing eight people. The National Transportation Safety Board, which is investigating the crash, has said the voice recorder on that plane was not working.

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Associated Press journalists Hannah Schoenbaum in Salt Lake City and Felicia Fonseca in Flagstaff, Arizona, contributed to this report.