In a jointly issued press release, the police agencies said they have reason to believe she is in danger.
"When last seen, Brittany was wearing the red sleeveless shirt seen in the below photo, blue jeans, and flip flops. She has braces on her teeth and typically wears her sunglasses on her head, as seen in the photo. Brittany is about 5'6 and 140 lbs.
We have reason to believe Brittany Thompson is endangered. Anyone with information about her whereabouts or about the man seen in this photograph should call Massachusetts State Police at 508-820-2121 immediately."
In an emailed message to parents Wednesday, Medfield Schools Superintendent Jeff Marsden said the girl is a student at Tri-County Regional Vocational Technical High School.
"There is no evidence that the girl was forcibly abducted. In addition, there is no evidence that this was a random act," he wrote. "We are confident that the community is safe so our dismissal and afterschool programs will go on as normally scheduled.
He continued: "Although the girl is no longer a student in the Medfield Public Schools, she is a part of our community and we all share the concern for her well being. We will continue to work closely with the Medfield Police Department and will update you as needed."
Medfield Police have posted an alert on Mass Most Wanted for information on an unidentified man seen leaving the town library Monday with a 17-year-old girl who has a developmental disability.
The girl has not returned home since, said Police Officer Michelle Manganello. She was last seen about 4 p.m. Monday, leaving the library with the man. Police have the library video, and from that, submitted a photo of the man to Mass Most Wanted.
"We don't have any identification on him," Manganello said Wednesday.
At 6:32 p.m. Monday, the girl's mother reported her missing to Medfield Police, explaining that her daughter was not at the library when she went there to pick her up at 5 p.m.
The male, shown in the photo still from the library video, is described as being heavy set with a light complexion and of medium height. He has brown hair.
Medfield police posted the alert on Mass Most Wanted, a public information website, on Tuesday morning, when the teen had not returned home.
Anyone with information on the man or the girl should contact officer Manganello at (508) 359-2315.