At least 27 people, including 18 children, were shot and killed at an elementary school in Newtown, Conn., according to multiple reports coming out of the city.

Tweets from the Associated Press and CBS News report that 27 people have been killed in the shooting, which would make it the worst K-12 school shooting in American history. According to initial reports, 18 of the dead are children.

The Hartford Courant also reports that an entire classroom is still unaccounted for since the school was evacuated.

NBC News reports that the assailant was a 24-year-old Connecticut man named Ryan Lanza. He was found dead at the school, according to multiple reports. Lanza's mother, a teacher at the school, is reportedly among the victims.

According to Connecticut Gov. Dannel Malloy's Twitter feed, he has arrived on the scene in Newtown. He also spoke with President Obama and other federal officials about any federal assistance that the city might need in the shooting's wake.

This is a breaking news story. We will continue to update it throughout the afternoon.