Basketball star Kobe Bryant, his 13-year-old daughter Gianna and seven other people were killed Sunday morning when their helicopter crashed into a fog-covered hillside near Calabasas, California, while on the way to a youth basketball tournament.

The death of Bryant, 41, stunned fans around the world who had watched him join the NBA as a confident teenager and depart a five-time champion with the Los Angeles Lakers. After retiring in 2016, he won an Oscar and coached Gianna’s basketball team.

Investigators are exploring several possible causes of the crash, including mechanical failure, and questions remain about what went wrong for an experienced pilot who had frequently flown similar routes in Southern California. But here’s what we know about the tragic flight.

Witnesses said the helicopter flew through clouds and fog.

In his last message before the crash, the pilot told air traffic control he was climbing to avoid a cloud layer. Witnesses on the ground reported seeing the helicopter flying through a layer of clouds and fog seconds before it crashed.

Although the flight took off in clear skies in Orange County, it eventually encountered a fog so thick that it nearly blinded drivers on the freeway. Visibility was so poor that the Los Angeles Police Department had grounded its fleet of helicopters.

The helicopter fell at a rate of about 23 mph and slammed into the hill in a “high-energy impact crash,” according to the National Transportation Safety Board. It was also traveling forward about 152 mph just before it crashed, according to radar data.

It is not clear why the helicopter fell at such a quick rate, which experts said was too fast for a standard landing. The aircraft slammed into the hillside at 1,085 feet after climbing to 2,300 feet; it may have missed clearing the top of the hill by 20 to 30 feet.

Investigators said the helicopter lacked a key warning system.

Officials have said that the helicopter was not outfitted with a system to warn pilots if they are getting too close to the ground, technology that is voluntary but has been recommended by the NTSB for more than a decade. Despite the safety advantages, an air safety consultant said, companies tend “to view it as a cost not required by regulation.”

A terrain awareness and warning system can help prevent crashes, especially when pilots have limited visibility, by providing a detailed image of surrounding terrain and triggering auditory and visual warnings. The systems are required on many fixed-wing aircraft.

It is too early to know whether the lack of a warning system played a role in Sunday’s crash. The helicopter was not carrying a cockpit voice recorder, but an iPad and cellphone were discovered in the wreckage.

The helicopter company was not certified to fly in poor visibility.

The pilot flying the helicopter did not have the legal authority to navigate with his instruments because the aircraft owner did not have the necessary federal certification, according to three sources familiar with the charter helicopter company’s operations.

Island Express Helicopters, which owned the helicopter, had a Federal Aviation Administration operating certification that limited its pilots to flying under what are known as visual flight rules, or VFR, with at least 3 miles of visibility and a cloud ceiling no lower than 1,000 feet above the ground. The company did not have certification for its pilots to fly with instruments.

The helicopter had sophisticated instruments onboard that in other circumstances would allow for instrument flight, and the pilot was certified to fly by them. But because of the company’s certification limitations, he was required to fly only in conditions of sufficient visibility to navigate visually.

The luxury helicopter has a reputation as a safe aircraft.

The Sikorsky S-76B helicopter that was carrying Bryant is a model widely used for executive and VIP passenger transportation, for search-and-rescue work and as an air ambulance. The manufacturer said that 10 countries use the helicopters to transport their heads of state; the one that crashed had once been owned by the state of Illinois.

Federal aviation accident records list eight accidents involving S-76Bs over 26 years before 2020, two of them involving deaths or serious injuries to people onboard.

The experienced pilot flew high-profile clients.

Ara Zobayan, the 50-year-old pilot, learned to fly in 1998, earned a commercial license in 2007 and was certified to teach others, according to the company that trained him and federal records. He had logged more than 8,200 hours of flight time, officials said, and at least 1,250 in the Sikorsky S-76B.

Zobayan had no accidents or enforcement actions on his record, according to the FAA. He flew Bryant regularly and had made the same trip the day before the crash, roughly 90 miles from John Wayne Airport to the Camarillo Airport, near Bryant’s Mamba Sports Academy.

The pilot was a familiar face to other high-profile clients, including celebrity Kylie Jenner and Kawhi Leonard, who plays for the Los Angeles Clippers. “He was one of the best pilots,” Leonard said. “That’s a guy who you ask for to fly you from city to city.”