"He made a point of getting to know students, asking them about details of their lives, enthusiastically sharing his own experiences. He brightened the lives of all of us, and words cannot describe the loss we feel."
Fellow club member Andrew Ding told of the car crash during a drive to New Hampshire, describing how Collier showed no hint of anger or exasperation over the damage to his vehicle.
"Sean was the first out of our car to go check and see if the (other driver) was OK," Ding wrote in an email to CNN, adding that "it would be exceedingly difficult to imagine him not stepping up and doing the right thing when he had the chance, which unfortunately he did last night."
Such awareness and devotion were evident in college, said Kristen Kuehnle, who chairs the department of criminal justice at Salem State University, where Collier graduated with honors in 2009 as a criminal justice major.
Kuehnle remembered that Collier got an 'A' in the "Women in Criminal Justice" class she taught, recalling him as "bright" and well-rounded with a "great sense of humor that you really need."
"When you're graduating with honors, you've demonstrated ability to be thoughtful and look at all perspectives," she said.
Describing Collier as a model candidate for police work, she said he had "the vision that we are looking for in our students."
"He always wanted to be a police officer," Kuehnle said. "That was pretty clear, but it wasn't like the whistles and bells and toys. It was about really wanting to make a difference and being good at what you do."
That's the impression he left on those who knew him at MIT.
"Sean is not the stereotypical cop who is kind of intimidating, but rather a friendly and down-to-earth kind of guy that you would want to be friends with," David Hou, an MIT sophomore, told CNN.
To Ding, Collier was "the kind of guy you'd ask to watch your wallet."
Ben Artin, another EMS volunteer, told how Collier proposed arranging a social event to help broaden the relationship between the campus police and the emergency service workers.
"Sean was rare in the degree to which, while working for MIT PD, he wanted to socially engage with the students," Artin wrote to CNN, adding that "I hope that we follow through on this proposal as we heal."