SAN FRANCISCO (KRON) -- For opening statements of the Bob Lee murder trial Monday, prosecutors dubbed the defendant, Nima Momeni, as a "wannabe tough guy big brother," and portrayed him as a knife-wielding murderer. Momeni is charged with murdering Lee on April 4, 2023 under San Francisco's Bay Bridge. Lee was an executive for cryptocurrency company MobileCoin and the founder of Cash App. He spent his last day alive partying with Momeni's younger sister, Khazar, hanging out in hotel rooms, and drinking at a private club known as The Battery, attorneys said. Momeni, 40, was convinced that something "inappropriate" had happened between his sister and the tech mogul, prosecutors told the jury. On the afternoon of April 3, 2023, Lee left Khazar at a drug dealer's apartment and returned to his hotel room with a friend to talk about new tech ideas, Assistant District Attorney Omid Talai said. Around 9 p.m., Momeni called Lee to interrogate him, Talai said, and the phone call was "aggressive, awkward, (and) strange." Homicide victim Bob Lee Momeni demanded to know from Lee, "What was happening with my sister over there? What inappropriate things were going on? What about the girls getting naked?" Talai said. Khazar stands in the Hall of Justice in San Francisco on May 18, 2023. (AP Photo / Godofredo A. Vásquez) Talai told the jury, "It's clear from this phone call, the defendant is this overprotective wannabe tough guy who wants some answers. Bob is being Bob ... trying to calm the defendant down." Evidence presented at trial will show Momeni's motive to murder Lee was "obvious," and it centers on Khazar, the prosecutor said. Talai began his opening statements by telling the jury, "Stabbed in his heart and left to die. Robert Lee, known as Bob, just a 43 year old man, was driven to a dark secluded area after 2 a.m. and was violently stabbed. He immediately began bleeding profusely and began taking his last breaths. Ladies and gentlemen, you are sitting in court with a murderer. A coward who stabbed an unarmed man three times." Nima Momeni walks into the courtroom for his arraignment in San Francisco on May 2, 2023. (Gabrielle Lurie /San Francisco Chronicle via AP, Pool, File) Before Khazar found out that Lee was dead, she sent an "invaluable text" message on April 4, 2023, Talai said. Her text to Lee was displayed on screen for jurors. It stated, "Just wanted to make sure your doing ok. Cause i know nima came wayyyyyy down hard on you. And thank you for being such a classy man handling it with class." "These were texts six hours after the murder," Talai told the jury. Surveillance cameras from Khazar's Millennium Tower apartment building recorded Lee and Momeni leaving together at 2 a.m. on April 4, 2023. Momeni secretly took a knife from his sister's kitchen and hid it under his clothing, prosecutors said. Khazar is seen in the San Francisco courthouse in 2023. (Getty Images) "The night seems done. But the defendant does not drive Bob home (to 1 Hotel). He takes him here on the 400 block of Main Street. Bob ends of dying about a block away," Talai said. The prosecutor admitted to jurors that the only surveillance video that shows the actual stabbing is extremely grainy. Two figures in the video are too pixelated to identify. As Lee stumbled away from the homicide scene, he left a trail of blood. "Just like that trail of blood found all the way down the block, (SFPD Detective Brent Dittmer) starts to follow a massive trail of evidence left behind by our coward of a defendant. What do police find on other side of that CalTrans fence? The murder weapon ... a kitchen knife. There's blood on the blade," Talai told jurors. Hours after the homicide, Momeni sent a text message to his sister talking about about "rape." Khazar replied in text messages that she was not raped by Lee. She wrote, "Blow (cocaine) messed up your mind. It makes you act lunatic." Defense attorney Saam Zangeneh's opening statement portrayed Lee as a man who went on a 90-hour-long drug bender around San Francisco as he and friends hopped from private parties, to bars, to nightclubs, to hotel rooms. "His phone records, his text messages, and testimony of witnesses will show he's awake from 8 a.m. Friday until 2:30 on Tuesday morning. It's not because he's an insomniac. It's because this period of time was fueled by substance abuse, drugs, mostly cocaine," Zangeneh told jurors. Zangeneh said Momeni had "no motive" to kill Lee, and the district attorney's case will "fall apart" once jurors hear all the evidence. The high profile trial will stretch over three months, the judge said. https://twitter.com/AmyLarson25/status/1845905087782105476 Zangeneh claimed that Lee invited Momeni to go to the Gold Club, a strip club, on the night of April 3. "Does that sound like bad blood? Does that fit the narrative (from prosecutors)? This motive they are trying to tell you, it doesn't exist," the defense attorney said. A bombshell was pitched by Zangeneh during his opening statement about why Lee and Momeni left Khazar's Millennium Tower apartment together. "Khazar kicks them out in a stealthy way. She text both guys, 'I'm tired and I want to go to sleep.'" Khazar wanted to get rid of Lee and her brother because her alleged drug dealer, Jeremy Boivin, was coming over, the defense attorney said. "She wanted to keep partying and she wanted the guy who had all the drugs. Jeremy told (police) that he stayed there for days. Khazar has a husband. The husband walked in on Jeremy in bed with Khazar," Zangeneh told jurors. After Lee and Momeni left Khazar's apartment, they drove in Momeni's BMW to a street under the Bay Bridge. "Around 2:30 a.m., after a 90-hour drug fueled bender, Bob Lee took a knife out of his pocket ... to attack Nima," Zangeneh said. Cocaine caused Lee to become paranoid, aggressive and violent, the defense attorney told jurors. Momeni had a defensive stab wound on his hip, he added. Zangeneh said after Lee pulled out the knife and thrust it at Momeni, Momeni fought back for his life. Zangeneh finished his opening statements by saying, "Nima Momeni was forced to defend himself, forced to stand his ground. Once we fill in the gaps, the only viable verdict in this case is not guilty. It's terrible somebody's dead, nobody likes that, but you have the right to defend yourself. You have the right to self defense." Krista Lee talks to reporters on August 1, 2023 in the San Francisco courthouse. (KRON4 photo / Amy Larson) Defense attorneys fought to have Lee's ex-wife banned from the courtroom for the trial by accusing her of intimidating witnesses. The ex-wife allegedly calls Khazar "the whore of high tower," according to a motion filed in court. As soon as Judge Alexandra Gordon denied the defense team's motion Monday morning, Lee's ex-wife entered the courtroom flanked by 10 additional family members dressed in black. Lee's family sat in the front row of the courtroom gallery. Tim Oliver Lee, brother of Bob Lee (left); Rick Lee, father of Bob Lee (second from left); and Krista Lee, ex-wife of Bob Lee (fourth from left) enter the courtroom on October 14, 2024 in San Francisco. (Photo by Lea Suzuki /San Francisco Chronicle via Getty Images) Before calling on their first witness, prosecutors played a heart-wrenching 911 call that recorded Lee's last words. "Help, help, help, help, help," Lee begs to an emergency dispatcher. The dispatcher replied, "Sir, where are you?" Lee says, "Somebody stabbed me. Help me, please." Lee was recorded by surveillance cameras during the 911 call. He lifted up his shirt to see blood spilling from his torso. "Please help me. I'm standing on the street. I need to get to the hospital," Lee told the dispatcher. On the tape, Lee begins moaning in agonizing pain. His voice starts to trail off and becomes fainter as he begs, "Help, help, help. Please, please." Sirens can be heard in the background of the tape just before the 911 call ends. https://twitter.com/AmyLarson25/status/1845903070477119824 Lee's two teenaged children listened to opening statements, but they remained out of the courtroom gallery while the tape was played. Lee's father was held his hands up to his face as he listened to his son dying. Prosecutors called on San Francisco Police Department Officer Joseph Rinaldi as their first witness. "There was an adult male on the ground in front of 403 Main Street. He was bleeding," Rinaldi testified. Photographs were displayed for the jury showing officers standing around Lee as he was laying unconscious on the pavement in a pool a blood. Khazar will testify on the witness stand on Tuesday, according to attorneys.