Newtown is about 60 miles northeast of New York City and the school has about 600 students.
NBC Connecticut further reported that the entire school district was in lockdown and police and FBI agents were going room to room searching the school.
Such cases have prompted several politicians to seek a review of American gun laws. In August, a gunman went on a shooting spree in a Wisconsin gurdwara, claiming six lives.
In July, 12 people were killed and 38 injured in Colorado where a gunman opened fire at the premiere of the Batman movie 'The Dark Knight Rises.
Image: Gov. Dannel P. Malloy, right, talks with officials at a staging area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., about 60 miles (96 kilometers) northeast of New York City, Friday, Dec. 14, 2012. An official with knowledge of Friday's shooting said 27 people were dead, including 18 children. (AP Photo/Jessica Hill)