Bob Lee, the founder of payment app Cash App and chief product officer of cryptocurrency firm Mobile Coin, has died after being stabbed in San Francisco.

Lee’s father and the San Francisco police confirmed the death of the tech entrepreneur early on Tuesday morning in the city’s downtown area, which is known for its vibrant tech startup scene.

The police responded to a report of a stabbing on Main Street and found a 43-year-old man suffering from apparent stab wounds, who later died in the hospital.

Homicide detectives have opened an investigation into the incident without naming Lee. Tech leaders expressed shock and paid tributes to Lee following the news.

MobileCoin CEO Joshua Goldbard called Lee a “dynamo” and “force of nature,” while venture capitalist Ron Conway said the world had lost a “great innovator, intelligence, and spirit.” Twitter co-founder and Cash App developer Square, Jack Dorsey, called the killing “heartbreaking” and described Lee as “instrumental” to the payment system. Lee had also previously worked for Google, helping to develop its Android platform.