A spokesman for the National Transportation Safety Board told ABC News that a Sikorsky S-76 crashed on a hillside in the town of Calabasas around 10 a.m. The circumstances surrounding the crash are unknown, and an investigation is ongoing.
Initial reports indicated that five people were on board and killed in the crash. But at an afternoon press conference, Los Angeles County Sheriff Alex Villanueva said the passenger manifest indicated there were nine people on board - a pilot and eight passengers - and officials now believe that all nine died in the crash.
Bryant's daughter, 13-year-old Gianna, was also reportedly one of the passengers on board the aircraft, according to an ESPN source. They were on their way to a travel basketball game along with another player and parent. The other victims have not been identified.
Also killed in the crash was John Altobelli, a baseball coach in his 28th season at Orange Coast College. It is believed his wife and daughter were with him on the helicopter.
Another person killed in the crash was identified by friends and family as Christina Mauser, an assistant basketball coach at Harbor Day School in Orange County.
Her husband Matthew Mauser, the head basketball coach at the school, posted on Facebook: "My kids and I are devastated. We lost our beautiful wife and mom today in a helicopter crash. Please respect our privacy. Thank you for all the well wishes they mean so much."
Los Angeles County fire and sheriff's department officials say the crash was reported at 9:47 a.m. in rough terrain in the area of Las Virgenes Road and Willow Glen Street.
When firefighters arrived, they found a fire had started, consuming about a quarter-acre of brush. The fire was particularly difficult to fight because of the presence of magnesium, which is difficult to extinguish and reacts with oxygen and water, county Fire Chief Daryl Osby said.
Paramedics also helicoptered to the scene and were hoisted down to try to aid any possible survivors, but all died at the scene, Osby said.
Villanueva declined to officially confirm the identities of any who were killed on board the helicopter, awaiting official identification from the county coroner's office.
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The National Transportation Safety Board is sending a team of 18 investigators and staff members to investigate the cause of the crash. They will look at factors such as operations, human performance, air worthiness, structures and power plants. The FBI is assisting the NTSB with documenting evidence as part of a longstanding partnership between the two agencies, but it remains an NTSB investigation.
Los Angeles County Chief Medical Examiner Dr. Jonathan Lucas said because of the rough terrain in the area, recovery of the bodies could take several days.
Los Angeles County Sheriff Alex Villanueva asked the public to stay away from the area. He said the situation is a "logistical nightmare" because of the rough terrain and fans and well-wishers are crowding the neighborhood. The Federal Aviation Administration has secured a 5-mile flight restriction from the area of the crash.
Aviation experts describe the Sikorsky S-76 as a well-made aircraft that could seat at least a dozen people.
"The S-76 is a pretty expensive, sophisticated helicopter. ... It's certainly a quality helicopter," said Justin Green, an aviation attorney in New York who flew helicopters in the Marine Corps.
Green believes weather may have contributed to the crash. Pilots can become disoriented in bad weather, losing track of which direction is up. Green said a pilot flying an S-76 would be instrument-rated, meaning they could fly the helicopter without relying on visual cues from outside.
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.
Juan Bonilla of Calabasas said he was working on his roof Sunday morning when he heard a helicopter flying low nearby. He said he thought it was a sheriff's helicopter on a training mission. He heard nothing amiss with the engine or rotors and said he did not see any mechanical issue with the chopper. It was foggy Sunday morning, but he said visibility didn't seem to be low at the time of the crash.
The 41-year-old Bryant spent his entire professional basketball career with the Los Angeles Lakers and won five championships. Bryant was drafted to the NBA directly out of Lower Merion High School in Ardmore, Penn., a suburb of Philadelphia. He was the 13th choice in the first round of the 1996 NBA Draft.
Messages of condolence and tributes to one of basketball's all-time greats have been pouring in.
"Words can't describe the pain I'm feeling," Michael Jordan said in a statement. "I loved Kobe - he was like a little brother to me. We used to talk often, and I will miss those conversations very much. He was a fierce competitor, one of the greats of the game and a creative force. Kobe was also an amazing dad who loved his family deeply - and took great pride in his daughter's love for the game of basketball."
NBA Commissioner Adam Silver said in part: "But he will be remembered most for inspiring people around the world to pick up a basketball and compete to the very best of their ability. He was generous with the wisdom he acquired and saw it as his mission to share it with future generations of players, taking special delight in passing down his love of the game to Gianna."
Shaquille O'Neal tweeted: "There's no words to express the pain Im going through with this tragedy of loosing my neice Gigi & my brother @kobebryant. I love u and u will be missed. My condolences goes out to the Bryant family and the families of the other passengers on board. IM SICK RIGHT NOW."
Former Lakers coach Phil Jackson issued a statement: "The crash was a tragedy for multiple families. My heart goes out to Vanessa and the families that lost loved ones. Kobe was a chosen one - special in many ways to many people. Our relationship as coach/player transcended the norm. He went beyond the veil."
Clippers Coach Doc Rivers broke down when talking about Bryant.
"The news is just devastating to everybody who knew him," said Rivers, who coached the Boston Celtics against Kobe's Lakers in the 2010 NBA Finals. "And he means a lot to me, obviously. He was such a great opponent. It's what you want in sports. He had that DNA that very few athletes can ever have."
Dozens of people began to gather on an intersection near the site of the crash in Calabasas Sunday afternoon, as well as at the Staples Center in downtown Los Angeles, where Bryant played for years.
Bryant, a.k.a. Black Mamba, was the fourth-highest scorer in NBA history, with 33,643 points, recently removed from third by LeBron James. Bryant retired in 2016, finishing two decades with the Lakers as a prolific scorer with a sublime all-around game and a relentless competitive ethic.
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Los Angeles Mayor Eric Garcetti issued a written statement in response to the news, calling Bryant a "giant who inspired, amazed, and thrilled people everywhere."
"He will live forever in the heart of Los Angeles, and will be remembered through the ages as one of our greatest heroes."
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The Associated Press contributed to this report.