The founder of multibillion-dollar tech company Cash App was fatally stabbed near downtown San Francisco on Tuesday, US media report.
San Francisco police found a 43-year-old man with stab wounds and treated him before he later died in hospital.
CBS News, the BBC's US partner, identified the man as Bob Lee, also the ex-chief technology officer at Square.
San Francisco officials have been criticised for their response to a wave of violent crime in recent years.
The California's San Francisco Police Department said officers responded to reports of a stabbing on Tuesday at around 02:35 local time (10:35 BST).
They found Mr Lee unconscious on the ground with two stab wounds to his chest, according to the San Francisco Chronicle, and started to administer aid before rushing him to San Francisco General Hospital, where he died.
"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.
Mr Lee, 43, was the chief product officer of the cryptocurrency company MobileCoin, who released a statement on Wednesday saying that Mr Lee "passed away yesterday", without listing a cause of death.
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 prompted renewed criticism of violent crime in the Californian city.
Tesla founder 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 that the 12 homicides in San Francisco this year is a 20% increase compared to the same time period in the previous year, according to CBS.
San Francisco police have yet to identify a suspect in this case, and no arrests have been made in the ongoing investigation.
