His voice straining with emotion at times, former Minneapolis police officer Mohamed Noor broke his nearly two-year silence and testified Thursday that he fatally shot Justine Ruszczyk Damond because he feared she was a threat to his and his partner's lives.

"It felt like my whole world came crashing down," Noor said, recounting the moment when he realized that he had shot an unarmed civilian. "Great anguish. I had trouble breathing."

Noor's testimony bolstered the defense's argument that a loud bang on the officers' squad caused them to fear for their lives as Damond approached their vehicle in a south Minneapolis alley near midnight. It also opened the door for prosecutors to highlight discrepancies between Noor's testimony and other evidence, question the reasonableness of his actions and raise doubts about his credibility by attacking his memory, which ranged from highly specific on mundane details to completely blank on some key issues.

"You never saw her hands?" Assistant Hennepin County Attorney Amy Sweasy asked during her rapid-fire, aggressive cross-examination.

Noor, who largely remained calm and collected through 2 ½ hours of testimony, said he did not.

"I had to make a split-second decision," he said.

"You did not see anything in her hands?" Sweasy asked.

"No, ma'am," he said.

Noor, who resumes testimony Friday morning, told the court that he was in the passenger seat on July 15, 2017, as he and his partner, Matthew Harrity, drove down an alley in response to Damond's 911 call about a possible sexual assault behind her home.

Having identified no victim or suspect, he said, they were parked at the end of the alley about to leave when they heard a "loud bang" on the driver's side.

"Oh, Jesus!" Harrity yelled, according to Noor.

Harrity, whom Noor had described as typically "very calm," grabbed for his gun. But, Noor testified, it got caught in the holster.

"He turned to me with fear in his eyes," Noor testified.

Noor said he rose from his seat, pressed his left arm against Harrity's chest and saw a blonde woman in a pink T-shirt raising her right arm outside Harrity's open window.

"I fired one shot," he said. "The threat was gone. She could have had a weapon."

Dressed in a dark-colored suit and tie, Noor's even demeanor cracked when, in a strained voice, he recounted how he exited the squad to see that Damond was unarmed. Harrity told him to holster his gun, Noor said.

Damond took a few steps back as both officers helped her to the ground, he testified.

Noor said he felt "great pain" and experienced "slight paralysis" upon realizing that Damond was unarmed. Damond's stepmother, who watched the proceeding with Damond's father, brother, sister-in-law and fiancé, cried and expressed apparent disbelief.

The officers performed CPR on Damond, who was pronounced dead at the scene.

"Have you ever seen [Harrity] that scared before?" defense attorney Thomas Plunkett asked.

"No, sir," Noor said.

"What was your intent when you pulled that trigger?" Plunkett asked.

"My intent was to stop the threat and save my partner's life," Noor said.

Plunkett asked if the shooting comported with his reason to become a police officer — wanting to serve.

"No, sir," he said through tears. "If I knew this would happen I never would have gotten into law enforcement."

Under questioning by Plunkett, Noor also said he did not have his gun out when they drove down Damond's alley, which is contrary to the prosecution's version of events.

Noor's testimony marked the first time he has spoken publicly about the night Damond was killed. In the wake of the shooting he repeatedly declined to speak with investigators and a 2018 grand jury investigating the incident. He has met with a defense investigator and expert witness who are expected to testify Friday.

Sweasy was stern in her cross-examination of Noor, escalating her tone and volume throughout while Noor remained unfazed, addressing her in a softer, more deferential tone than one he used with Plunkett.

"What was foremost in your mind — her call or your officer safety plan?" Sweasy asked.

"We always think about officer safety," Noor said, later adding, "I relied on my training."

The prosecution's use-of-force expert witnesses previously testified that no use of force was called for that night, and Sweasy raised questions Thursday about whether Harrity's reaction was enough to justify Noor's response.

"How do you know what officer Harrity saw?" she asked.

"I can read my partner," Noor said. "I've worked with him long enough to know when he fears for his life."

"Does 'Oh, Jesus' mean, 'I fear for my life?' " she asked.

"No, ma'am," Noor said, repeating that Harrity looked "directly at me" as he struggled to unholster his weapon in reaction to the noise on the squad.

Sweasy drilled Noor about what he saw beyond Harrity's reaction, asking if someone raising an arm and doing "nothing more" justified the use of deadly force. Harrity previously testified that he had been startled by a "thud" on the driver's side, saw a "silhouette" outside his window and drew his weapon out and held it by his rib cage.

"Coupled with my partner's actions, yes," Noor said. "I had to make a split-second decision."

Sweasy asked if Noor saw a weapon on Damond, or whether Harrity called out for help or warned him about a gun.

No, Noor answered each time.

"Her whole blonde hair, pink T-shirt and all that was all threat to you?" Sweasy asked.

Defense attorneys focused heavily on Noor's training as a Minneapolis police cadet and its focus on officer safety and teamwork to build the framework for their argument that Noor acted in his and Harrity's best interest when he shot Damond.

After getting through the background process, he enrolled in the 29-week police academy, which he described as an intensive blend of classroom lectures and scenario-based training on principles ranging from "defensive techniques" and "ground fighting" to "takedowns" and firearms training.

During a weeklong period toward the end of the academy training — known as "Officer Survival Week" — cadets are assigned uniforms, a squad car and furnished with a blue dummy gun and called on to respond to simulated 911 calls. In one such scenario, the cadets were told to respond to a fake domestic scenario, involving an armed person.

Noor described being "shot" in one scenario, in which he was required to respond to a call about shots fired. He said he ran toward the sound of gunshots without considering his surroundings.

"So what happens" if you're slow to react?" Plunkett asked.

"You die," Noor said. "It's just drilled in you by repeating the scenarios."

Noor testified that some of the training covered what to do if caught in a squad car during an ambush. Noor said that officers were taught to "improvise," in order to protect their lives or that of their partner.

"As long as your partner is safe, you can shoot out of your squad in any direction," he said.