Four people died Tuesday when a medical transport plane crashed and caught fire on the Navajo Nation in northern Arizona, the tribe said in a statement.

The crash involving the CSI Aviation company out of Albuquerque, New Mexico, occurred near the airport in Chinle, about 200 miles (321 kilometers) northeast of Flagstaff. Those on board were medical personnel who were on their way to a hospital to pickup a patient.

The Beechcraft 300 crashed in the early afternoon while landing at the airport, Federal Aviation Administration officials said in an email. The National Transportation Safety Board and the FAA are investigating.

The tribe said the cause of the crash is unknown.

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.

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