The founder of multibillion-dollar tech company Cash App, Bob Lee, was fatally stabbed near San Francisco city centre on Tuesday, his family has said.
Police found a 43-year-old man with stab wounds and treated him before he later died in hospital.
His father, Rick Lee, confirmed his son - who was also the ex-chief technology officer at Square - had been killed.
San Francisco officials have been criticised for their response to a wave of violent crime in recent years.
The latest crime data suggests the city is one of the most-dangerous in the US.
'I lost my best friend'
Police officers responded to reports of the stabbing on Tuesday at around 02:35 local time (10:35 BST).
The San Francisco Standard viewed CCTV footage that shows Mr Lee walking down a deserted alleyway, seemingly looking for help.
Mr Lee is seen stumbling towards a parked car and lifting up his shirt to reveal his wound, but the vehicle drives off before the tech entrepreneur falls to the ground.
Police found Mr Lee unconscious in the Rincon Hill neighbourhood with two stab wounds to his chest, and started to administer aid before rushing him to hospital, according to the San Francisco Chronicle.
Mr Lee's father posted a message on Facebook on Wednesday saying: "I just lost my best friend, my son Bob Lee when he lost his life on the street in San Francisco early Tuesday morning."
Mr Lee was living in Miami, Florida, but he was visiting San Francisco for a leadership summit and was staying on to see friends, according to BBC's US partner, CBS News.
Tim Oliver Lee, Mr Lee's brother, also posted on Facebook: "He really was the best of us. I was so fortunate to grow up with him, and I feel like I've lost part of myself."
At his time of death, Mr Lee was the chief product officer of the cryptocurrency company MobileCoin.
"He was a generous decent human being who didn't deserve to be killed," said Bill Barhydt, CEO of Abra, a cryptocurrency company, on Twitter, noting that Mr Lee also was a father.
Cash App is a smartphone-based payment app that allows person-to-person money transfers and is now worth $40bn (£32bn), according to Forbes.
Since launching in 2013, its user base has skyrocketed, hitting seven million monthly active users in 2017, and climbing to 30 million in 2020.
Mr Lee's death has prompted renewed criticism of violent crime in the Californian city.
Tesla boss and Twitter chief executive Elon Musk responded to tributes to Mr Lee, saying: "Violent crime in [San Francisco] is horrific and even if attackers are caught, they are often released immediately."
Data from 2021 shows that residents there face a 1-in-16 chance each year of being a victim of property or violent crime, according to the Hoover Institution, a policy research think tank - making the city more dangerous than 98% of US cities.
Homicides have been a particular issue for San Francisco since the pandemic.
There were 56 homicides in the city in 2021 and 2022, and preliminary police data shows there have been 12 homicides in San Francisco so far this year.
San Francisco police have yet to identify a suspect in this case, and no arrests have been made in the ongoing investigation.
Brooke Jenkins, San Francisco district attorney, tweeted a tip line number and wrote "we do not tolerate these horrific acts of violence in San Francisco".
San Francisco Mayor London Breed said in a statement that "the homicide of Bob Lee is a horrible tragedy" and sought to assure residents the city was "prioritising public safety".