Lee’s death at age 43 — after staggering on a deserted downtown San Francisco street seeking help — 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 a unique set in his sister’s condo, drove Lee to a secluded area and stabbed him three times, then fled.

“Stabbed through his heart and left to die,” said Omid Talai, assistant district attorney, “our victim was stabbed repeatedly, once in his chest, once in his hip and literally one puncturing his heart. Robert Lee. Known as Bob.”

Defense lawyers disagree, and they say that Lee, high on drugs, attacked Momeni. They plan to make their opening statement after lunch.

“Our theory is that Bob had the knife, and that Nima acted in self defense,” attorney Saam Zangeneh told The Associated Press last week.

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.

On Monday, Momeni was seated with his lawyers, wearing a dark suit.

His mother, who has been a steadfast presence at hearings, was in court alone Monday morning. On the other side of the courtroom sat members of Lee’s family, including his ex-wife, father and brother.

Talai, the assistant district attorney, said jurors will hear from a friend of Lee’s, who spent time with him and Momeni’s sister the day before Lee was stabbed.

The friend will testify that Momeni angrily grilled Lee on the phone that night over his sister, drugs and “girls getting naked,” acting like “an overprotective, wannabe tough guy” while Lee was being mellow and happy, Talai said.

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.

Talai said jurors will see video of Lee and Momeni leaving the building after 2 a.m. and driving off together in Momeni’s car. Other video will show the two men getting out of the car in a isolated spot by the Bay Bridge, and then Momeni stabbing Lee three times, tossing the knife from his sister’s kitchen set and quickly driving away.

Talai said the prosecution will share text messages in which Momeni, the following morning, tells his sister he did not know what happened to Lee that night.

The lawyer said video recorded by a San Francisco police detective trailing Momeni before his arrest shows him reenacting the three stabbing motions outside his previous lawyer’s office, but no reenactment of a struggle over the knife that his attorneys say Lee wielded first.

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

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.

He called 911 as he staggered, bleeding out and begging dispatch for help, Talai said.

Family members for Momeni and Lee declined comment Monday.

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

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