As the Tsarnaevs headed toward the MIT campus, Collier was on duty near Kendall Square in Cambridge and DiFava pulled his car next to Collier’s cruiser.
In the days after the Marathon bombings, the MIT chief had ordered additional security for the campus. And Collier, who began work on the MIT force in early 2012, was nearing the completion of his 3 to 11:15 p.m. shift.
About 9:30 p.m., Collier was on routine patrol. He was parked by the corner of Vassar and Main streets. It was a spot where motorists would sometimes take a chance, making an illegal shortcut through campus to avoid a red light.
“We ask patrols to sit there,’’ DiFava explained. It prevents the forbidden cut-throughs and it provides a high-profile presence for the MIT community.
What are you doing? the chief asked his young officer.
“Just making sure everybody behaves,’’ Collier told him.
The two men chatted easily for several minutes. And then DiFava pulled away.
The chief was home for perhaps a half hour when his phone rang. “It was the deputy chief,’’ DiFava said. “He said Sean Collier has been shot.’’
In the fog of what would become a deadly and dangerous night, there were initial reports that Collier had been responding to a disturbance. That turned out to be wrong.
What actually happened was more cold-blooded, authorities said. Police officials have called it an assassination, an execution.
Authorities say video from a surveillance camera shows the suspects approaching Collier’s car from the rear as he sat in his cruiser. Collier was shot five times, including twice in the head, officials said.
“He didn’t stand a chance,’’ DiFava said.
The two bombing suspects allegedly tried to steal Collier’s weapon, but they couldn’t unlock it.
“The retention holster does its job well, so perhaps they didn’t get the gun because of that holster,’’ the MIT chief said. “Maybe that’s what thwarted them from getting the gun, because the gun was not removed from the holster.’’
Authorities say it is not clear why the Tsarnaevs were at MIT or why they targeted Collier.
“We have all kinds of unanswered questions,’’ DiFava said.
Last week, the Somerville Police Department asked the city’s Board of Aldermen to honor Collier, appointing him posthumously to the city’s police force.
As tributes flooded in about Collier, including those from Biden, DiFava could scarcely utter words about his fallen officer.
“From the time I saw him to the time he was dead, it was probably about an hour,’’ DiFava said.
If the details of the attack were unspeakable, they proved to be useful weapons for the alleged terrorists.
“I just killed a policeman in Cambridge,’’ Tamerlan Tsarnaev told the man he would meet next as he waved a silver handgun at him.
Thursday, 11 p.m.
It was a cool spring night, and a 26-year-old entrepreneur from Cambridge had picked the perfect way to unwind: take his new $50,000 Mercedes-Benz out for a spin.
As he drove his spacious 2013 black SUV toward Boston, the man — a Chinese immigrant who asked to be identified only by his American nickname, Danny — noticed a swarm of police cars with flashing blue lights near the MIT campus.
After a short drive across the river, Danny, who earned a master’s degree from Northeastern University last year and is trained as an engineer, pulled his car to the curb on Brighton Avenue to answer a text message.
Suddenly, an old sedan swerved behind him and slammed to a stop. A thin young man in dark clothes got out, approached the passenger window, and rapped on the glass. Danny lowered the window.
The man reached an arm through, unlocked the door, climbed in, and pointed a handgun.
He demanded cash, and made it all too clear this wasn’t a joke.
“You know about the Boston Marathon bombing? I did that,” Tamerlan said. “Don’t be stupid.’’
What followed was a harrowing 75 minutes during which 26-year-old Tamerlan Tsarnaev — soon joined in the car by his brother Dzhokhar — forced Danny on a circuitous journey through Brighton, Watertown, and back to Cambridge.
The brothers wanted more cash than the $45 that Danny had and they talked about driving to New York. Otherwise, their intentions were unclear. The odyssey took on a surreal tone. The brothers marveled at the features of the Mercedes-Benz, popped in a CD of what sounded like Middle Eastern religious music, and talked about credit limits for students.
Through it all, Danny feared they would kill him if he made one misstep. He frantically thought about how to escape. And he prayed.
They lapped Brighton and crossed the Charles River into Watertown, following Arsenal Street. Opening Danny’s wallet, Tamerlan asked for his ATM code.Continued...