Cash App Founder Bob Lee’s Killing Followed Visit With Accused Killer’s Sister

Nima Momeni stabbed Cash App founder Bob Lee to death after the pair left the apartment where Mr. Momeni’s sister lived, prosecutors alleged Friday.

While prosecutors wouldn’t say what occurred in the apartment, a court filing said the two men discussed the suspected killer’s sister earlier in the day and that Mr. Momeni had been angry with Mr. Lee.

San Francisco District Attorney Brooke Jenkins gave the most detailed account of Mr. Lee’s final hours alive in a motion to detain Mr. Momeni without bail filed Friday. Mr. Momeni was arrested on suspicion of murder Thursday in a case that has rocked Silicon Valley.

Ms. Jenkins didn’t reveal a specific motive for the killing, which occurred in the early morning hours of April 4. But much of the narrative her office presented revolved around Mr. Momeni’s younger sister, identified by Ms. Jenkins as Khazar Momeni.

Earlier Friday, a judge ordered that Mr. Momeni be held without bail while awaiting his arraignment, which was postponed until April 25 because the lead defense attorney is out of town.

Robert Canny, an attorney for Mr. Momeni, said the facts of the case will come out during the course of the trial. Mr. Canny didn’t respond to a request for comment on the filing Friday afternoon.

Ms. Momeni couldn’t be reached for comment. Her husband, a prominent San Francisco plastic surgeon, didn’t respond to requests for comment. The couple declined to speak with the media before Friday’s court hearing.

According to the filing, Mr. Lee and Ms. Momeni were drinking and talking with others on the afternoon of April 3 at a home in San Francisco, a witness told police. The witness said that Mr. Momeni’s sister “was married but the relationship had possibly been in jeopardy.”

Ms. Jenkins said that Mr. Lee was a friend of Ms. Momeni but declined to elaborate.

Mr. Lee and the witness left and went to the Cash App founder’s hotel room, where Mr. Lee and Mr. Momeni engaged in a phone conversation about Ms. Momeni. Mr. Momeni questioned Mr. Lee “regarding whether his sister was doing drugs or anything inappropriate,” the witness told police. Mr. Lee reassured Mr. Momeni that nothing inappropriate occurred, the witness said.

Later that night, Mr. Lee allegedly went to the Millennium Tower, an expensive San Francisco apartment building where Ms. Momeni lived. Surveillance video showed Mr. Lee, wearing black clothing, entering the building at 12:39 a.m., the filing said.

Mr. Lee and Mr. Momeni rode down the elevator at the Millennium Tower together at around 2:03 a.m. and got into Mr. Momeni’s white BMW Z4, according to the filing. The pair drove a short ways before getting out of the car.

The two figures stood together for about five minutes, after which Mr. Momeni stabbed Mr. Lee three separate times, including twice in the chest, prosecutors alleged.

“One of the stab wounds, showing a direct and clear intent to kill, penetrated [Mr. Lee’s] heart,” the filing said.

Mr. Momeni then threw down the kitchen knife he had used and drove away, leaving Mr. Lee to die, according to the court filing.

Investigators later unlocked Mr. Lee’s phone and found a text message from Ms. Momeni sent to Mr. Lee, prosecutors wrote.

“Just wanted to make sure your doing ok Cause I know nima came wayyyyyy down hard on you,” the message said.

Ms. Jenkins said her office is looking at the killing as a case of premeditated murder.

“This is a person who was in his vehicle with a kitchen knife,” said Ms. Jenkins. “That’s not something most of us carry around at all times with us—and so that this was something that he intended to do.”

Mr. Momeni lived in Emeryville, a small city across the Bay from San Francisco, according to police. A LinkedIn profile for a Nima Momeni of Emeryville showed he is a tech consultant and an entrepreneur.

Mr. Lee was the chief product officer at MobileCoin Inc., a cryptocurrency company, at the time of his death. He is known for creating Cash App, which people can use to transfer money, buy crypto and trade stocks. He made the app while he was the chief technology officer at Square Inc., now known as Block Inc., where he worked from 2010 to 2014.

Before he joined Square, he worked at Alphabet Inc.’s Google and helped develop the Android mobile operating system.

He worked with people across Silicon Valley as an executive and an angel investor. He invested in companies including SpaceX, run by Elon Musk, and Figma, the collaboration-software business Adobe Inc. agreed to acquire for $20 billion last year.

Mr. Lee’s old colleagues and friends responded to his death with stories about his selflessness and his knack for coding. He was known on Twitter by his handle “crazybob.”