Four people died when a hot air balloon crashed in Eloy, Arizona, on Sunday morning, police said.

A fifth person was in critical condition after the “tragic” crash, Eloy police said.

The balloon went down about 7:50 a.m. in the desert area of Sunshine Boulevard and Hannah Road, the police department said. Eloy is about 50 miles northwest of Tucson and 65 miles southeast of Phoenix.

The cause of the crash remains unknown, according to the department. The National Transportation Safety Board and the Federal Aviation Administration were on the scene and took over the investigation.

The names of the victims are being withheld until family members are notified, the police department said. The agency added that more information will be released as it becomes available.

“The Eloy Police Department extends its deepest condolences to the families and friends of those involved in this heartbreaking incident,” the department said.