Bryant's helicopter flew in fog that grounded other choppers

The helicopter carrying Kobe Bryant and eight others that crashed into a rugged hillside outside Los Angeles was flying in foggy conditions considered dangerous enough that local police agencies grounded their choppers.

The helicopter plunged into a steep hillside at about 9:45 am Sunday with an impact that scattered debris over an area the size of a football field and killed all aboard.

The accident unleashed an outpouring of grief from admirers around the world who mourned the sudden loss of the all-time basketball great who spent his entire 20-year career with the Los Angeles Lakers.

Thousands of fans, many wearing Bryant jerseys and chanting his name, gathered outside the Staples Center in downtown Los Angeles, home of the Lakers and site of Sunday's Grammy Awards where Bryant was honoured.

The 41-year-old Bryant, who perished with his 13-year-old daughter, Gianna, was one of the game's most popular players and the face of the 16-time NBA champion Lakers.

The cause of the crash was unknown, but conditions at the time were such that the Los Angeles Police Department and the county sheriff's department grounded their helicopters.

The Los Angeles County medical examiner, Dr Jonathan Lucas, said the rugged terrain complicated efforts to recover the remains.

He estimated it would take at least a couple of days to complete that task before identifications can be made.

Bryant's helicopter left Santa Ana in Orange County, south of Los Angeles, shortly after 9 am and circled for a time just east of Interstate 5, near Glendale.

Air traffic controllers noted poor visibility around Burbank, just to the north, and Van Nuys, to the northwest.

After holding up the helicopter for other aircraft, they cleared the Sikorsky S-76 to proceed north along Interstate 5 through Burbank before turning west to follow US Route 101, the Ventura Highway.

Shortly after 9:40 am, the helicopter turned again, toward the southeast, and climbed to more than 2000 feet (609 meters).

It then descended and crashed into the hillside at about 1400 feet (426 meters), according to data from Flightradar24.

When it struck the ground, the helicopter was flying at about 160 knots (184 mph) and descending at a rate of more than 4000 feet per minute, the data showed.

The chopper went down in Calabasas, about 30 miles (48 kilometers) northwest of downtown Los Angeles.

Bryant's Mamba Sports Academy in nearby Thousand Oaks was holding a basketball tournament on Sunday.

Federal transportation safety investigators were on their way to the scene.

Among other things, they will look at the pilot's history, the chopper's maintenance records and the records of its owner and operator, said NTSB board member Jennifer Homendy at a news conference.

Kurt Deetz, a pilot who used to fly Bryant in the chopper, said the crash was more likely caused by bad weather than engine or mechanical issues.

The likelihood of a catastrophic twin engine failure on that aircraft it just doesn't happen, he told the Los Angeles Times.

Justin Green, an aviation attorney in New York who flew helicopters in the Marine Corps, said pilots can become disoriented in low visibility, losing track of which direction is up.

Green said a pilot flying an S-76 would be instrument-rated, meaning that person could fly the helicopter without relying on visual cues from outside.

The National Transportation Safety Board typically issues a preliminary report within about 10 days that will give a rough summary of what investigators have learned.

A ruling on the cause can take a year or more.

Colin Storm was in his living room in Calabasas when he heard what sounded to him like a low-flying airplane or helicopter.

"It was very foggy so we couldn't see anything, he said.

But then we heard some sputtering and then a boom.

The fog cleared a bit, and Storm could see smoke rising from the hillside in front of his home.