When authorities first reported Chism missing on Tuesday, people responded on the police department's Facebook page with prayers for his "safe return." One neighbor described him as "a polite and friendly boy."
The tenor of the comments changed significantly once he was linked to Ritzer's killing.
"I'd say he's less 'polite and friendly' than you thought," wrote one Facebook commenter. "Everybody has to keep their eyes open. It's no longer safe to say that you can take everybody at face value."
'The sweetest, most harmless person ever'
Ritzer graduated from Assumption College in 2011, that school said on Twitter. She was pursuing a master's degree in school counseling at Salem State University, that school said.
"She believed children have much to offer and often do not realize how special they are as individuals," the university said. "In her application to Salem State she said she was dedicated to 'helping students in times of need.'"
Charlotte Dzerkacz in 2011 taught at the same middle school with Ritzer, during which time the two became close friends.
At all times, Ritzer wore a wide smile and was approachable to students and colleagues alike.
"She was energetic, she was compassionate," Dzerkacz said. "You couldn't ask for anything more from a teacher or a friend."
Like Dzerkacz, Ritzer's aunt Shirley Martellucci said her niece never had any trouble with students -- including during the past two years, when she was at Danvers High.
"She always wanted to be a teacher, all her life," Martellucci said. "It's just unbelievable that someone would take her life at such a young age."
On her Twitter account, Ritzer interspersed homework assignments and exhortations to work through tough math problems with cooking talk and inspirational messages.
"No matter what happens in life, be good to people," she wrote in August. "Being good to people is a wonderful legacy to leave behind."
Her family issued a statement seeking privacy as they mourn their "amazing, beautiful daughter and sister."
"Everyone that knew and loved Colleen knew of her passion for teaching and how she mentored each and every one of her students," the family said in the statement.
Students at Danvers High echoed that sentiment, as well as that of her school district describing Ritzer as "a dynamic and brilliant ray of light."
In the halls, she was guaranteed to offer a warm smile. In class, she could be counted on to give an appreciated pat on the back. Even once the final bell sounded, she could always be counted on to offer teaching, guidance and support.
"She was always the teacher (who would) go the extra mile for students, always the teacher to be there for students after school," said Kyle Cahill.
"It's just crazy."