The tragic intertwining of these four lives began around 10:20 Thursday night, when, according to law enforcement officials, the police at the Massachusetts Institute of Technology received reports of gunfire on the Cambridge campus. Responding officers soon found their colleague, an M.I.T. officer, Sean A. Collier, 26, dead from multiple gunshots after possibly being ambushed in his police vehicle.
As investigators were determining that two men — believed to be the suspects, Tamerlan Tsarnaev, 26, and his brother, Dzhokhar, 19 — had shot Officer Collier, word came from another part of Cambridge of a carjacking of a Mercedes-Benz sport utility vehicle at gunpoint by two men who would release its owner a half-hour later.
Within two hours, some of the officers who had responded to the shooting at M.I.T. were racing to Watertown, about five miles to the west, where the local police had tried to pull over the carjacked vehicle. In the ensuing shootout, the older Mr. Tsarnaev was shot dead, and a Massachusetts Bay Transportation Authority police officer, Richard H. Donohue, 33, was seriously wounded.
Throughout the night and into the day, most of the nation focused its attention on the search for the younger Mr. Tsarnaev, a dragnet that had the Boston area in a nerve-jangling lockdown. Meanwhile, friends and family members of the two officers mourned and worried on the sidelines of an unfolding international event.
Officer Collier was a compact man with a crackling intellect who seemed born to be a police officer. “People come into police work for a lot of reasons,” the M.I.T. police chief, John DiFava, said. “He was one of those who came on because it was really what he was meant to do with his life.”
Sean Collier grew up in Wilmington, a leafy town of about 22,000 people less than 20 miles north of Boston. After high school, he graduated from Salem State University with a degree in criminal justice and eventually began working as an information-technology employee for the Police Department in Somerville, a congested blue-collar city hard against Cambridge.
According to Somerville’s mayor, Joseph A. Curtatone, everybody in the city seemed to know Mr. Collier, even though he was a kid from Wilmington. In addition to vastly improving the Police Department’s Web site, he immersed himself in the community, volunteering, for example, with the Somerville Boxing Club, a youth-outreach program, the mayor said.
“It’s like he was a lifer here,” Mr. Curtatone said.
But he yearned to shed civilian garb for a police uniform. When Mr. Collier applied for a job with the M.I.T. police, Chief DiFava had already received high recommendations for this young man from the chief’s neighbor, a Somerville deputy police chief, and a cousin, a Somerville police officer.
In January 2012, Mr. Collier joined the ranks of nearly 60 M.I.T. police officers, all armed with semiautomatic pistols. Their job is to keep safe a small city of 11,000 people, many of them foreign graduate students who, Chief DiFava said, “come from places where the cops are not their friends.”
But the chief said Officer Collier won over many students, in part by joining the Outing Club, whose members hike, ski and explore the New England outdoors.
“And I’ll tell you, they loved him,” Chief DiFava said.
That affection is reflected in many online reminiscences from students who recalled his easygoing but protective nature. Among the outdoors crowd, he was known for his willingness to yodel, for sharing his pepperoni snacks, and for making the most of every moment, as when he wrote a note inviting people to hike a part of the White Mountains on the anniversary of an ascent of Mount Everest by the Polish mountaineer Krzysztof Wielicki:
“… so feel free to bring any Polish dishes, wear Poland’s colors (red and white), bring a Polish flag (because you know you have one laying around your apartment), or just actually be from Poland (cool!) to commemorate this awesome feat.”
Dina Kraft contributed reporting from Cambridge, Mass.