Updated at 5:45 p.m.: Revised throughout to reflect Monday's testimony.

In the days after a former Farmers Branch cop killed a teen and seriously wounded another, he searched the internet to find out when it's legal to kill a burglar, prosecutors said.

Off duty and out of uniform, Ken Johnson caught two teens breaking into his SUV last year at his Farmers Branch apartment complex. They drove off, and Johnson got in his vehicle and chased them into Addison.

Surveillance video shows the Chevy Tahoe hitting the back of the teens' Dodge Challenger, causing it to spin out of control. After the Challenger crashed, Johnson fired 16 times into the car, killing 16-year-old Jose Cruz and seriously wounding Edgar Rodriguez, who was also 16 at the time.

The murder trial for Johnson, 37, started Monday and detailed the March 2016 car chase and deadly shooting. Johnson is also charged with aggravated assault in the shooting of Rodriguez.

Ken Johnson resigned from the Farmers Branch Police Department.

Johnson told investigators that he feared for his life before firing into Cruz's car and said there was an "altercation."

Prosecutor Jason Hermus told jurors Monday they were going to see "physical evidence that shows he's a liar."

Defense attorney Robert Rogers said Johnson announced he was a police officer and saw Cruz reaching for something inside the car.

"They're doing adult things. They're stealing from hard-working people," Rogers said. "Ken is doing what we expect police officers to do: to follow criminals."

The teens were unarmed, and Rodriguez testified Monday that he didn't hear Johnson say anything before he started shooting.

"I saw the guy outside the window with the gun pointing at us. As soon as I saw him, he started shooting," Rodriguez said. "He just started shooting without saying anything."

Rodriguez, now 18, was shot several times in the head and hands. He held up his right hand in the courtroom to show he was missing his middle finger.

Rodriguez described stealing seats from the Tahoe and driving off with Cruz. The teens didn't realize they were being followed until Johnson rammed their car.

After they crashed, Rodriguez asked Cruz if he was ready to go to jail.

Jose Cruz was shot and killed in March 2016.

"I thought [Johnson] was going to call the police and we were going to get arrested," Rodriguez said.

Moments before the shooting, Cruz called for his friend, "Edgar, Edgar," with fear in his voice, Rodriguez said. Then he heard the gunshots.

Rodriguez tried to scramble into the back seat, holding his hands over his head. A bullet lodged into his cellphone, which Rodriguez clutched over his head. He said it's what saved his life.

He didn't learn his friend had been killed until the next day.

Officers arrived minutes after the shooting, and Johnson told them he was a cop. He was dressed in red shorts and a T-shirt. He hadn't grabbed his phone, police radio, badge or handcuffs before leaving his apartment to chase the teens.

"If he wants to make a peaceful arrest, he didn't even grab the means to achieve that," Hermus said during opening statements. "He had already committed to shooting every one of those bullets into those kids, and he did."

Phone records show that in the days after the shooting Johnson searched the internet for information on when it's legal to kill someone to defend property and whether it's possible to see through tinted windows, Hermus said.

The windows on Cruz's Challenger were darkly tinted, and prosecutors suggested Johnson wouldn't have been able to see whether the teen was reaching for anything.

Crime scene photos of Cruz's body show his hands in his lap "palms up like they had just fallen off the steering wheel," said Addison Detective David McDonald.

Family members in the courtroom quietly sobbed as photos of Cruz's bloody body were shown.

His mother, Ana Henriquez, testified briefly Monday about her oldest son, calling him her "right arm."

The teen went to high school in the morning and worked in the afternoons. Cruz would cook for his siblings and help care for his younger brother, who has special needs.

"He killed my dreams," Henriquez said of Johnson. "I want justice for my son."

Johnson resigned from the Farmers Branch Police Department weeks after the shooting. The police chief said Johnson violated department policies when he chased the teens and rammed them with his private vehicle.

"That's just something we don't do," Chief Sid Fuller said at the time.

Cruz's family has sued Johnson and the city of Farmers Branch for wrongful death.

The officer previously had been accused twice of using excessive force during an eight-year stint with DART police. Both times, he was cleared.

The trial is expected to continue at least a week. If convicted, Johnson faces up to life in prison.