Updated at 11:20 p.m. ET
DANVERS, Mass. A well-liked teacher was found slain in woods behind this quiet Massachusetts town's high school, and a 14-year-old boy who was found walking along a state highway overnight was charged with killing her.
Blood found in a second-floor school bathroom helped lead investigators to the body ofwho was reported missing when she didn't come home from work on Tuesday, Essex District Attorney Jonathan Blodgett said.
"She was a very, very respected, loved teacher," Blodgett said.
The suspect, Philip Chism, was arraigned on a murder charge Wednesday and ordered held without bail. The teenager,and pleasant, also did not come home from school on Tuesday and was spotted walking along Route 1 in the neighboring town of Topsfield at about 12:30 a.m. Wednesday.
"We have no reason to believe that any other suspects are involved," Blodgett told reporters during a Wednesday morning news conference.
Officials didn't release a cause of death and haven't discussed a motive in the killing.
A court filing said Ritzer and Chism were known to each other from the high school, but it did not elaborate. The arrest was made based on statements by the suspect and corroborating evidence at multiple scenes, investigators said in court documents.
Ritzer's family said they are mourning the death of 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 a statement provided by her uncle Dale Webster.
Riley Doyle, 14 and a freshman in Ritzer's geometry class, told CBSNews.com that her teacher "was always really upbeat and positive and excited about math."
"She made every lesson like you wanted to learn it," Riley said. "For the first time, math became one of my favorite classes."
Riley said Ritzer's students thought highly of her.
"She was always very courteous to her students, and she would never talk down to them," Riley said. "She treated them like people."
Riley's sister Regan Doyle, 17 and a senior at the high school, told CBSNews.com that she didn't have Ritzer as a teacher, but she had "never heard one bad thing about her."
"My sister would always come home from school saying how she was her favorite teacher," Regan said.
Regan said Ritzer's death came as a shock to the school.
"It's just a tragedy, and it's so bad because she's so young," Regan said, "and you could tell that she really loved her job and the kids, and it's sad that this had to happen."
At his arraignment in adult court in Salem, Chism's defense attorney, Denise Regan, argued for the proceeding to be closed and her client to be allowed to stay hidden because of his age. The judge denied the request but granted Regan's request for the teen to be given a mental-health evaluation. A not-guilty plea was entered on Chism's behalf and he was ordered held without bail.
The tall, lanky teenager had moved to Massachusetts from Tennessee before the start of the school year and was a top scorer on the school's junior varsity soccer team, said Kyle Cahill, a junior who also plays soccer. He said the team had been wondering where Chism was when he skipped a team dinner Tuesday night.
"We're all just a family. It just amazes me really," Cahill said. "He wasn't violent at all. He was really the opposite of aggressive."
Ritzer had a Twitter account where she gave homework assignments, encouraged students and described herself as a "math teacher often too excited about the topics I'm teaching."
She was a 2011 graduate of Assumption College in Worcester, a school spokeswoman said Wednesday. She graduated magna cum laude with a bachelor of arts degree in math, a minor in psychology and a secondary education concentration, according to the college's 2011 commencement program.
"I just remember seeing her in the hallway and she always had a smile, like 'hi' to everyone, and she was just an amazing person," Megan Papagelis, a Danvers High graduate, told CBS Boston station WBZ-AM.
"It's just shocking to see that this could happen," Papagelis told WBZ-AM. "Unbelievable."
One of her former students, Chris Weimert, 17, said she was a warm, welcoming person who would stand outside her classroom and say hello to students she didn't teach. He said she had been at the school for two years.
"She was the nicest teacher anyone could ever have. She always had a warm smile on her face," he said.
Ritzer liked to post homework assignments and words of encouragement on Twitter. In August she tweeted: "No matter what happens in life, be good to people. Being good to people is a wonderful legacy to leave behind."
Ryan Kelleher, a senior, said students related to the young teacher, who liked to wear jeans and UGG boots just like the teenagers she taught. Kelleher, who also plays soccer, said the arrest of the soft-spoken Chism didn't make sense to him.
"From what I know about him and seeing him every day, it just doesn't add up that he would do such a thing, unless this was all an act to fool somebody," the 17-year-old said.
Ritzer lived at home with her 20-year-old brother and her sister, a high school senior. The close-knit family was often outside, barbecuing, spending time together and enjoying each other's company, neighbors said.
Mary Duffy has lived next door to the Ritzers in the suburban neighborhood in Andover since the family moved there more than two decades ago. She had known Colleen Ritzer from the time she was a baby and said the Ritzers' oldest child had just one life ambition: to be a high school math teacher.}
"All I ever heard is that she loved her job," Duffy said.
All public schools in Danvers, about 20 miles north of Boston, were closed Wednesday.
Hundreds of people gathered Wednesday night to mourn Ritzer at a candlelight vigil in the high school parking lot. Many wore pink, the slain teacher's favorite color.
Classes were canceled at Danvers High until further notice, but grief counselors will be available at the school from 10 a.m. to noon on Thursday. School will be held as normal for kindergarten through Grade 8.
Ritzer is the second teacher allegedly killed by a student in the U.S. this week. A Sparks, Nev., middle school teacheron Monday.