After roughly 90 terrifying minutes as a captive of the Boston Marathon bombers, Dun Meng saw his chance. One of the men who had carjacked him had gone inside a Cambridge convenience store. The other was fiddling with a GPS device, his gun tucked into the car door next to him. Meng calculated his escape.  He counted down in his mind: 1, 2, 3, 4. . .  “This was the most terrifying moment, the most difficult decision in my life,” he told a federal jury in South Boston Thursday. “I dashed onto the street. I could feel he was trying to grab me.”  Meng, who made it safely to another convenience store across the street, recounted his ordeal on the sixth day of testimony in the trial of Dzhokhar Tsarnaev, who faces the death penalty for his role in the bombings and the killing of an MIT police officer.  Meng’s escape was a turning point in the manhunt for the bombers. Once they fled in Meng’s Mercedez Benz, authorities used the car’s tracking system to trace them to Dexter Avenue in Watertown. Police from several agencies descended on the brothers there, and a firefight ensued.  The moment also catapulted an unassuming Meng, a Chinese national and Kendall Square entrepreneur who had earned a master’s degree at Northeastern University, into a central role in the weeklong narrative of the bombings and the hunt for the suspects. His story was first told to the Globe on April 26, 2013, in which he was referred to by one of his American nicknames, Danny.  Tamerlan Tsarnaev, 26, was killed during the firefight in Watertown. His younger brother escaped, but was found hours later hiding in a boat parked in a backyard a few blocks away.  Dun Meng (far right) is depicted in a courtroom sketch testifying in the trial of Dzhokhar Tsarnaev.  In court earlier Thursday, testimony focused on the killing of MIT police Officer Sean Collier three days after the Marathon bombings. Anthony Grassi, a Cambridge police detective, testified that he was called to the MIT campus just after 10:20 p.m. on April 18, 2013, for a report of an officer down. There, he found officers from MIT and Cambridge tending to Collier on the ground next to his police cruiser.  “He was bleeding profusely, from his face, his head,” Grassi said.  An image from an ATM machine shows Dzhokhar Tsarnaev as he withdrew $800 from the bank account of Dun Meng.  He reviewed the scene and found three shell casings on the ground behind Collier’s cruiser, and three more inside, on the passenger seat and floor.  “You could see a puddle of blood under the driver’s seat,” Grassi testified. Collier had been sitting in his cruiser when he was shot. He was found slumped to his right, and authorities say the Tsarnaev brothers tried unsuccessfully to remove his gun from his holster.  Dr. Renee Robinson, a state medical examiner, told jurors that the 26-year-old Collier was shot three times in the head, including once between the eyes, and three times in the hand, and he would have died instantly. Robinson said the shots were fired at close range, at least one with the muzzle pressed against his skin, based on the pattern of the bullet’s entry wound.  One juror broke into tears, and another wiped his eyes with a tissue, as they looked at autopsy photos. Another juror took copious notes.  On Wednesday, the jury heard testimony from a graduate student who said he saw Tsarnaev at Collier’s cruiser in the moments after the officer was shot.  Tsarnaev’s lawyer has admitted her client’s involvement in the Marathon attacks and the shooting of Collier, but — with prosecutors seeking the death penalty — he has pleaded not guilty, and his lawyers have sought to use the trial to show that his older brother was the mastermind. The lawyers are building an argument that Tsarnaev should be spared a death sentence because, they say, he was less culpable.  For instance, defense lawyers allege, Tamerlan Tsarnaev was the one who shot and killed Collier. And, they argue, the older brother planned the bombings.  David Bruck, one of Tsarnaev’s lawyers, questioned Meng to show that Tamerlan Tsarnaev was the one who carjacked Meng and pointed a gun at his face. Meng testified that he never saw the younger Tsarnaev with a gun.  In their first encounter since the carjacking, Meng and Tsarnaev exchanged a quick glance as Meng took the witness stand, but neither showed any emotion.  For about two hours Thursday, Meng recounted his ordeal. A partner in a mobile application company, Meng said he left work about 10:30 p.m., and “I decided to drive along the Charles River, to relax.”  “I didn’t really have a destination,” he said.  Sitting at the edge of his seat, he described how he took several turns, and pulled his car to the side of the road on Brighton Avenue to send a text message to a friend. Suddenly, a sedan pulled up behind him quickly. A man stepped out of the passenger side of the car, approached his passenger side window and tapped. He asked him to lower the window. Deng thought he was going to ask for directions, but the man instead reached inside and opened the door, stepping inside his car.  “He pulled the gun to me, to my head,” Meng told jurors, describing how he thought he was being robbed. The man pulled out his magazine, to show he had bullets.  Dun Meng is pictured escaping from his carjacked Mercedes in an image from a video surveillance camera at a Shell gas station.  “I’m serious, don’t be stupid,” the man told him.  Then he said, “Do you know the Boston Marathon explosions? I did it, and I just killed a police officer in Cambridge.”  Deng described how the man, who he later identified as Tamerlan Tsarnaev, directed him to drive into Watertown, down dark streets, toward Waltham, back to Boston, and then around. The driver of the sedan followed him. At one point, they stopped in Watertown and Tamerlan and the driver of the sedan, who Meng identified in court as Dzhokhar Tsarnaev, loaded items into the back of Meng’s Mercedes. He did not look.  Then, Dzhokhar Tsarnaev climbed into the back seat of his Mercedes, and Tamerlan Tsarnaev ordered Meng to get into the passenger seat. They stopped at a gas station, but it was closed. They stopped at a Bank of America ATM on Main Street in Watertown, where Dzhokhar Tsarnaev used Meng’s debit card to withdraw $800 in cash. Video footage of Tsarnaev making the transaction was played in court Thursday.  At a second stop at the sedan in Watertown, Tsarnaev grabbed a CD to play in Meng’s car that had what he described as “weird” religious music he had never heard before.  Surveillance image from Shell gas station shows Dzhokhar and Tamerlan Tsarnaev.  Meng thought of fleeing, but then thought better of it. There were two people in the car now, and Tamerlan Tsarnaev had said they both had guns. He didn’t think he could outrun them.  But at the Shell gas station on Memorial Drive, Meng found his opportunity. Dzhokhar Tsarnaev went in to pay for gas, and surveillance footage shows him inside the store picking up other items, including Doritos, and Red Bull.  Tamerlan Tsarnaev remained in the car, but he was playing with a GPS that Deng believes they grabbed at the second visit to the sedan. They had said something about wanting to go to New York. Meng wasn’t sure that he believed Tamerlan when he told him he would not kill him.  So on the count of four, Meng made a dash, racing into a Mobil gas station across the street and asking a clerk for help.  Tamerlan Tsarnaev was seen on surveillance footage getting his younger brother from inside the Shell station, and they drove away.  Meng hid in a storage room of the Mobil, and called police.  “Please, come help me, they have guns,” he said. “They did the Marathon explosions.”  102 hours in pursuit of the Marathon bombing suspects  MIT’s fallen officer built enduring connections
In April 2013, Dzhokhar Tsarnaev and his older brother Tamerlan allegedly set off two bombs that killed three and injured hundreds, shot and killed MIT police Officer Sean A. Collier, and carjacked a Chinese businessman — but refused to shoplift from a Cambridge gas station.