A United Airlines flight was diverted last week after a mysterious object struck the aircraft’s windshield at an altitude of 36,000 feet, causing a crack and injuring the pilot, according to the airline and U.S. aviation officials.

United Airlines stated that the Boeing 737-MAX 8, carrying 134 passengers and six crew members, which departed from Denver en route to Los Angeles, ultimately landed safely in Utah “to address the damage to its multi-layered windshield.” From the resulting debris, the pilot was injured and received treatment following the emergency landing. Meanwhile, the cracked windshield is being sent to the National Transportation Safety Board’s laboratory to help solve the mystery.

The airline also said that the passengers were transferred to another plane headed to Los Angeles later that same day, and United is working with its team to return the aircraft to service.

“This is an extremely unusual situation, given that the glass caused injury to the cockpit occupants and considering the altitude of 36,000 feet at which the impact occurred. That’s really the big mystery,” said ABC News aviation analyst John Nance. Aircraft windshields are designed with multiple layers to withstand damage from things like bird strikes, weather conditions, or debris, but experts say bird strikes at such high altitudes are rare.

“The aircraft collided with an object, and a cockpit window broke, so we had to make an emergency landing in Salt Lake City,” said student and passenger Heather Ramsey to ABC News, recalling the pilot’s message.