Boston (CNN) Dun Meng knew to pull over his Mercedes-Benz SUV to respond to a text message that night in April 2013. That's when a man got out of another car, jumped into his SUV and pointed a gun at him.

"He asked me, 'Do you know the Boston Marathon explosion?' " Meng told a crowded courtroom Thursday, speaking in halting English. An interpreter who spoke Meng's native Mandarin sat next to him but was not needed.

"He asked, 'You know who did it? I did it and I just killed a policeman in Cambridge,' " Meng said a man later identified as Tamerlan Tsarnaev told him.

Meng was on the witness stand in the death penalty trial of admitted Boston Marathon bomber Dzhokhar Tsarnaev, Tamerlan's younger brother. He recalled seeing the flashing lights of police cars in Cambridge earlier that night.

"After that, how did you feel?" Assistant U.S. Attorney William Weinreb asked.

"Terrified. The whole world is looking for him at the time. ... I thought it was just a typical robbery."

'Officer down'

There was nothing typical about the days after the Boston Marathon bombings, which killed three people and hurt more than 240 others.

Prosecutors say Dzhokhar and Tamerlan Tsarnaev had earlier killed Massachusetts Institute of Technology police Officer Sean Collier because they wanted his gun. But their efforts to take it were thwarted by a safety holster.

Dzhokhar Tsarnaev, who was 19 at the time, does not dispute he was present when Collier was killed on the evening of April 18, 2013, nor does he deny that he participated in the bombings three days earlier. He is being tried on 30 charges -- 17 of which carry the possibility of the death penalty.

Tamerlan Tsarnaev, 26, would not survive the night. He was killed in a chase and gunbattle with police that began with reports of an "officer down" at MIT.

It was after Collier's killing that Meng encountered the Tsarnaev brothers.

At the time, he was working on his master's degree at Northeastern University. Meng, a partner at a company that created a food delivery app, recalled feeling tired after work. He drove along the Charles River with no destination in mind. He pulled over after receiving a text message.

'Don't be stupid'

"I'm a traffic engineer, and I know it is unsafe to text while I'm driving," he said.

Photos: Boston Marathon bombing evidence Photos: Boston Marathon bombing evidence A jury condemned Dzhokhar Tsarnaev to death on Friday, May 15, for his role in killing four people and wounding hundreds more in the 2013 Boston Marathon bombings. See photos that were released as evidence in his trial. Hide Caption 1 of 35 Photos: Boston Marathon bombing evidence This undated photo of a young Tsarnaev with his brother, Tamerlan, was shown by the defense in the sentencing phase of the trial. Tamerlan died after being shot by police and run over by a car driven by his brother in the massive manhunt that followed the bombings. Hide Caption 2 of 35 Photos: Boston Marathon bombing evidence Katie Russell met Tamerlan Tsarnaev at a nightclub and dropped out of college to marry him. Her mother, Judith Russell, testified that Tamerlan came between Katie and her family and that Katie became isolated. She eventually converted to Islam and changed her name to Karima Tsarnaeva. She was the breadwinner. But when company came for dinner, she cooked, served the men and then retired to another room. Hide Caption 3 of 35 Photos: Boston Marathon bombing evidence This collection of photos of Dzhokhar Tsarnaev in his wrestling days was introduced by the defense. Hide Caption 4 of 35 Photos: Boston Marathon bombing evidence This photo of Tamerlan Tsarnaev, recovered from his computer, was shown during the sentencing phase. Hide Caption 5 of 35 Photos: Boston Marathon bombing evidence This image shows victims' positions in the crowd prior to the Boston Marathon bombings on April 15, 2013. Hide Caption 6 of 35 Photos: Boston Marathon bombing evidence Tsarnaev "flips the bird" in a jail cell during his first arraignment on July 10, 2013. The image was presented to jurors in the sentencing phase of his trial. Hide Caption 7 of 35 Photos: Boston Marathon bombing evidence Tsarnaev poses in front of a black standard adopted by various militant Islamist groups in this Instagram photo that was entered as evidence. Hide Caption 8 of 35 Photos: Boston Marathon bombing evidence Prosecutors say Tsarnaev was a self-radicalized jihadist who pored over militant writings, including the article "How to Build a Bomb in the Kitchen of Your Mom." It was found on his laptop and other devices, part of a full-edition download of Inspire magazine, a glossy English-language propaganda tool put out by al Qaeda in the Arabian Peninsula. Hide Caption 9 of 35 Photos: Boston Marathon bombing evidence This Russian manual on how to fire a handgun was found in the apartment where Tsarnaev's brother, Tamerlan, lived. Tamerlan Tsarnaev was killed in a shootout with police in Watertown, Massachusetts, on April 19, 2013. Hide Caption 10 of 35 Photos: Boston Marathon bombing evidence This copy of The Sovereign, which calls itself the "newspaper of the resistance," was also found in Tamerlan Tsarnaev's apartment. Hide Caption 11 of 35 Photos: Boston Marathon bombing evidence Pictured here is a box of bullets found on a street after the shootout in Watertown. The brothers' fingerprints were on the box, prosecutors said. Hide Caption 12 of 35 Photos: Boston Marathon bombing evidence A pressure cooker was embedded in the side of a resident's Honda during the Watertown shootout. Hide Caption 13 of 35 Photos: Boston Marathon bombing evidence Photos of the Watertown shootout were entered into evidence. Neighbors came to their windows and then retreated. One grabbed his infant son and headed toward the back of his house with his wife. Another grabbed a camera and took photographs from an upstairs window. Hide Caption 14 of 35 Photos: Boston Marathon bombing evidence The Tsarnaevs had carjacked a Mercedes SUV in Watertown before the shootout. The vehicle was covered in bulletholes, and the rear window was shattered. Hide Caption 15 of 35 Photos: Boston Marathon bombing evidence This unexploded pipe bomb was found at the scene of the shootout between police and the Tsarnaev brothers in Watertown. Hide Caption 16 of 35 Photos: Boston Marathon bombing evidence Prosecutors said these boards were attached to the boat where police found Dzhokhar Tsarnaev hiding. A carved message reads, "Stop killing our innocent people and we will stop." Hide Caption 17 of 35 Photos: Boston Marathon bombing evidence Smashed phones and an ATM card owned by carjacking victim Dun Meng were in the yard where Dzhokhar Tsarnaev was found. Hide Caption 18 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this surveillance image shows Tsarnaev visiting an ATM hours before a police chase and chaotic shootout in which more than 200 rounds were fired. Hide Caption 19 of 35 Photos: Boston Marathon bombing evidence Another view of Tsarnaev's visit to the ATM. Hide Caption 20 of 35 Photos: Boston Marathon bombing evidence Exhibits related to the shooting death of MIT Officer Sean Collier were introduced to the jury on Wednesday, March 11. This image from the crime scene appears to show a bloody gun. Hide Caption 21 of 35 Photos: Boston Marathon bombing evidence This burned tank top and yellow hoodie belonged to bombing survivor Jessica Kensky. Hide Caption 22 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this Fox Racing logo was from one of the backpacks containing a bomb. Hide Caption 23 of 35 Photos: Boston Marathon bombing evidence Prosecutors say this still image from surveillance video shows Tsarnaev in the UMass Dartmouth gym the day after the bombings. Hide Caption 24 of 35 Photos: Boston Marathon bombing evidence Prosecutors showed the jury photos of what they say are Tsarnaev's writings inside the boat he was captured in. Hide Caption 25 of 35 Photos: Boston Marathon bombing evidence This image is from a surveillance camera outside the Forum restaurant in Boston's Copley Square just after the bombing. Hide Caption 26 of 35 Photos: Boston Marathon bombing evidence Prosecutors presented two Twitter accounts linked to Tsarnaev that, they said, showed targeting the marathon had been on his mind for at least a year. One account, @J_tsar, contained 1,100 tweets and was the more mainstream of the two. On the day of the 2012 Boston Marathon, a tweet from the account read, "They will spend their money & they will regret it & they will be defeated." Hide Caption 27 of 35 Photos: Boston Marathon bombing evidence Prosecutors said the second Twitter account is evidence that Tsarnaev led a double life. By day, he was a slacker college sophomore. By night, he was a wannabe jihadist, posting on the account @Al_firdausiA. In one tweet, he urged people to listen to radical cleric Anwar al-Awlaki's lectures. "You will gain an unbelievable amount of knowledge," he said in March 2013, just weeks before the bombings. Prosecutors also allege in an indictment that Tsarnaev downloaded al-Awlaki's writings, calling him a "well-known al Qaeda propagandist." Al-Awlaki had been killed in a U.S. drone strike in 2011. Hide Caption 28 of 35 Photos: Boston Marathon bombing evidence Hide Caption 29 of 35 Photos: Boston Marathon bombing evidence A Boston Marathon bombing victim is tended to in the street. Hide Caption 30 of 35 Photos: Boston Marathon bombing evidence Victims at the finish line just after the bombing. Hide Caption 31 of 35 Photos: Boston Marathon bombing evidence Eight-year-old Martin Richard, the youngest victim , can be seen standing on the rail in the front row. Hide Caption 32 of 35 Photos: Boston Marathon bombing evidence A closer view of 8-year-old Martin Richard in the crowd before the bombing. Hide Caption 33 of 35 Photos: Boston Marathon bombing evidence Boston police tend to a wounded child. CNN has chosen not to show the young victim's face. Hide Caption 34 of 35 Photos: Boston Marathon bombing evidence Mayhem along Boylston Street. Hide Caption 35 of 35

A car pulled up quickly and stopped. A man in the car walked over. Meng said he thought the man wanted directions. Instead, the man pulled open the SUV door and climbed in.

Meng said Tamerlan Tsarnaev demanded cash. He said he had about $45 on him.

"He said, 'That's not enough. Where's your wallet?' So I give him my wallet, but there's no cash in the wallet."

The man pulled the magazine out of his handgun. He showed Meng it was loaded.

"I'm serious," Meng testified that the man told him moments before admitting to being the Boston Marathon bomber. "So don't be stupid."

After he handed over the cash, Meng said the man ordered him to drive. He said Dzhokhar Tsarnaev followed in a Honda but later got in the backseat of the SUV.

"He ordered me to every direction," he said of Tamerlan Tsarnaev.

"We had a conversation. ... He asked me my name and he asked me where I'm from. First few minutes my hands were shaking and I was having trouble driving. I was very scared."

'Muslims hate Americans'

Meng said his name was Manny. The man laughed when he thought Meng said Nanny.

"I said I'm Chinese. He said, 'OK, you are Chinese. I am Muslim and Muslims hate Americans.' I responded, 'I'm Chinese. Chinese are very friendly toward Muslims.' "

At one point, the carjacker asked Meng for the PIN code on his bank card.

"I tell him the PIN code," he testified. "It started with 86, somebody's birthday. He asked me if I was born in 1986, and I said yes."

Meng testified that he asked Tamerlan Tsarnaev: "Are you going to kill me tonight? He told me, 'I'm not going to kill you. Just relax, man.' "

The witness pointed to Dzhokhar Tsarnaev as the man who later sat in the backseat behind him. The men played a CD as they drove around.

'Victim of something'

"I would say weird," he said of the music. "It sounded religious."

At one point, Dzhokhar Tsarnaev went into a gas station convenience store to pay for gas and load up on snacks, Meng testified. Tamerlan fiddled with a GPS device.

Meng unbuckled his seat belt, opened the SUV door and got away, dashing across the street to another service station. He never looked back. Meng begged a worker to call the police. He hid there until officers arrived.

It was "the most difficult decision in my life," he said.

"I could feel he was trying to grab me. His hand was so close to my left hand I could feel the wind."

Cambridge police Officer Michael Nickerson had to talk Meng out of a gas station storage room when he arrived.

"He was trembling," Nickerson testified. "He was very scared. You could tell he was a victim of something."