SAN FRANCISCO (AP) — The murder trial of a tech consultant in the stabbing death of Cash App founder Bob Lee begins Monday, a year and a half after the widely admired entrepreneur was found staggering on a deserted downtown San Francisco street seeking help.

Lee’s death at age 43 stunned the tech community, and fellow executives and engineers penned tributes to his generosity and brilliance. Lee was chief product officer of cryptocurrency platform MobileCoin when he died. He was a father to two children.

Prosecutors say Nima Momeni, 40, planned the April 4 attack after a dispute over his younger sister, Khazar, with whom Lee was friends. They say Momeni took a knife from his sister’s condo, drove Lee to a secluded area and stabbed him three times, then fled.

Defense lawyers disagree, and they say that Lee, high on drugs, attacked Momeni.

“Our theory is that Bob had the knife, and that Nima acted in self defense,” attorney Saam Zangeneh said.

He said his client is eager to tell his side of the story, but they haven’t decided whether Momeni will testify in his defense.

Momeni, who lives in nearby Emeryville, California, has been in custody since his arrest days after Lee died at a San Francisco hospital. Momeni’s mother has been a steadfast presence at court hearings, and he is close to his sister.

Prosecutors have said in court documents that a friend of Lee’s told homicide investigators they had been drinking with Momeni’s sister the day before the stabbing. The friend said Momeni later questioned Lee about whether his sister was doing drugs or otherwise engaging in inappropriate behavior, and Lee said she had not.

Surveillance video of Lee’s final night shows him entering the posh Millennium Tower downtown, where Momeni’s sister lives with her husband, a prominent San Francisco plastic surgeon.

Video then shows Lee and Momeni leaving the building together after 2 a.m. and driving off in Momeni’s car. Lee was found around 2:30 a.m. in the Rincon Hill neighborhood of San Francisco, which has tech offices and condominiums but little activity in the early morning hours.

Police recovered a knife with a 4-inch (10-centimeter) blade at the scene. Prosecutors said tests showed Momeni’s DNA on the weapon’s handle and Lee’s DNA on the bloody blade. But the defense said police should have tested the handle for fingerprints, namely Lee’s.

Momeni, who has pleaded not guilty, faces 26 years to life if convicted.

San Francisco Superior Court Judge Alexandra Gordon has told jurors the trial could last until mid-December.