Severe turbulence rocked a flight from Phoenix to Honolulu Sunday, seriously injuring 11 people in what a Hawaiian Airlines official called an isolated and unusual event.

Jon Snook, the airline’s chief operating officer, said the airline hasn’t experienced “an incident of this nature in recent history.” The flight was full, carrying 278 passengers and 10 crew members, he said during an afternoon news conference.

Jim Ireland, director of Honolulu Emergency Medical Services, said 20 people were taken to hospitals, including 11 people deemed to be in serious condition.

At least one person was reported to lost consciousness, but all patients were awake and talking when they arrived at hospitals, he said.

Patients suffered cuts, including to the head, as well as bumps and bruises. Some people were nauseous and vomited as a result of extreme motion, he said. Altogether 36 people received treatment.

“We are also very happy, and we feel fortunate, that there were not any deaths or other critical injuries. And we’re also very hopeful that all will recover and make a full recovery,” Ireland said.

Snook said three flight attendants were among those taken to hospitals.

Snook said there was some internal damage to the aircraft during the turbulence. The seatbelt sign was on at the time, though some of those injured weren’t wearing them, he said.

Thomas Vaughan, a meteorologist with the National Weather Service in Honolulu, said there had been a weather advisory for thunderstorms that included Oahu and areas that would have included the flight path at the time of the incident.

The airline was aware of the weather forecast and the unstable air and weather conditions, but had no warning the particular patch of air where the turbulence occurred “was in any way dangerous,” Snook said.

He didn’t know how much altitude the plane lost during the turbulence, saying that would be part of an investigation involving the National Transportation Safety Board. The plane’s flight data recorder would provide those details, he said.

The investigation would also address precisely what the passengers and crew were doing at the time, he said.

The Airbus A330-200 began its descent immediately after the turbulence and crew declared an emergency due to the number of injuries on board, he said. Air traffic controllers gave the flight priority to land.

Snook said he could only speculate whether some passengers hit their heads, but that was likely based on the injuries and the damage to cabin panelling.

“If you don’t have your seatbelt on, you stay where you are as the aircraft goes down and that’s how those injuries occur,” Snook said.

Most people associate turbulence with heavy storms. But the most dangerous type is so-called clear-air turbulence. The wind-shear phenomenon can occur in wispy cirrus clouds or even clear air near thunderstorms, as differences in temperature and pressure create powerful currents of fast-moving air.

Planes can sail into clear-air turbulence without warning.