ELOY, Ariz. (AP) — An “unspecified problem” with the balloon portion of a hot air balloon may have led to Sunday’s crash in the Arizona desert that left four people dead and another critically injured, federal authorities said Tuesday.

The National Transportation Safety Board said in a statement that investigators had not found any mechanical anomalies. They separated the balloon from the basket, which carries passengers, and “everything appears to be intact.”

“An electronic device that could have relevant flight information and a video camera have been sent to NTSB headquarters in Washington, D.C., for further analysis,” the agency said. The balloon’s maintenance records and information on the pilot’s flight experience have been collected.

Eloy police said the Federal Aviation Administration also was assisting in the investigation.

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Authorities said 13 people were aboard the Kubicek BB 85 Z hot air balloon when it took off. Eight were skydivers who exited the gondola before the crash around 7:45 a.m. Sunday in Eloy, about 65 miles (100 kilometers) southeast of Phoenix.

Eloy Mayor Micah Powell said witnesses told investigators that the balloon itself appeared deflated, with its material “just straight up and down” seconds before a hard impact in an empty field that serves as a drop zone for skydivers.

The dead included pilot Cornelius van der Walt, 37, of Eloy; and three passengers: 28-year-old Kaitlynn “Katie” Bartrom of Andrews, Indiana; 28-year-old Chayton Wiescholek of Union City, Michigan; and 24-year-old Atahan Kiliccote of Cupertino, California.

Authorities said a 23-year-old woman frpm the Phoenix suburb of Scottsdale remained hospitalized in critical condition Tuesday.

Van der Walt was the founder of Droplyne Hot Air Balloon Rides that operates in Arizona and Utah, according to the company’s website.

Droplyne conducts daily flights up to 10,000 feet (3,000 meters) in elevation from the Eloy area November to April and from Moab, Utah, during the spring and summer.

The website also said Droplyne was founded in 2017 and had “a perfect safety record.”

A call to the company Tuesday seeking comment wasn’t immediately returned.