Gavin Masak, who lives nearby, told CBS News of the moment the helicopter crashed.
"It wasn't exactly like an explosion sound but it was like a loud boom. It sounded like a helicopter, but it sounded like a jet, like it was loud, so I went inside and told my dad what was happening. So when I came out I saw smoke on the hill but it wasn't like a big, black cloud of smoke, it was just grey," he said.
Other eyewitnesses told celebrity news site TMZ they heard the helicopter's engine sputtering before it went down.
The LA County Sheriff's Department shared pictures from the scene of the accident, showing a fire truck and smoke emerging from the hills.
The US Federal Aviation Administration identified the crashed helicopter as a Sikorsky S-76. It said it was investigating the accident alongside the National Transportation Safety Board.
Officials are set to give further details at a press conference at 22:00 GMT.
Who was Bryant?
Bryant played his entire 20-year career with the Los Angeles Lakers. He retired in April 2016.
His achievements include being the 2008 NBA Most Valuable Player and two-time NBA Finals MVP. He was also two-time NBA scoring champion and a two-time Olympic champion.
He famously scored 81 points against the Toronto Raptors in 2006, the second-highest single-game total in NBA history.
He also won an Oscar for best short animated film in 2018 for Dear Basketball, a five-minute film based on a love letter to the sport he had written in 2015.
Bryant and his wife, Vanessa, have four daughters: Gianna, Natalia, Bianca and Capri.
Bryant was accused of sexual assault in 2003 by a 19-year-old woman working at a Colorado resort. He denied the allegation, saying the two had consensual sex. The case was dropped after the accuser refused to testify in court and a later civil suit was settled out of court.
What has the reaction been?
Tributes for the sports star poured in on social media following news of the crash.
Deron Williams, who won an Olympic gold alongside Bryant, said he was lost for words. He described Bryant as "the greatest competitor I have ever gone up against and played with."