Another person was found dead at 36 Yogananda St. in Newtown, sources told The Courant.
The shooter is dead inside the school, and the situation is secure, said State Police Lt. J. Paul Vance.
Students described being ushered from their classrooms hand-in-hand, with their eyes closed, to the safety of a nearby fire station as police converged on the school.
There were conflicting reports about the identity of the shooter. The state police have not identified him.
Several news outlets, including The Associated Press and CNN, initially identified the shooter as Ryan Lanza and said his younger brother was being held for questioning as a possible second shooter.
The Associated Press is now reporting that the suspect is Adam Lanza, the younger brother, and that the older brother is being questioned.
CNN is no longer identifying the shooter.
A law enforcement official said the boys' mother, Nancy Lanza, works at the school as a teacher. The Associated Press is reporting that she is presumed dead.
The official also said Ryan Lanza's girlfriend and another friend are missing in New Jersey.
Another official, speaking on condition of anonymity because the investigation was still under way, said the gunman apparently had two guns. A law enforcement official in Washington said one of the guns was a .223-caliber rifle.President Obama, in an emotional address to the nation Friday afternoon, said "These neighborhoods are our neighborhoods, and these children are our children."
“I know there’s not a parent in American who doesn’t feel the same overwhelming grief that I do," he said. "The majority of those who died today were children, beautiful little kids between the ages of 5 and 10 years old. … They had their entire lives ahead of them. Birthdays, graduations, weddings, kids of their own."
“Among the fallen were also teachers, men and women who devoted their lives to help their children achieve their dreams,” he said.
Like other parents around the country, Obama said, “This evening, Michelle and I will ... hug our children a little tighter and tell them that we love them … but there are families in Connecticut who cannot do that tonight.”
“While nothing can fill the space of a lost child or a loved one, all of us can extend a hand to those in need, to remind them that we are there for them, that we are praying for them, that the love they felt for the ones they have lost will endure in their memories but also in ours.”
Public records show that Ryan Lanza lived at 36 Yogananda St. at one point, and he is also listed as living at 1313 Grand St. in Hoboken, N.J. Police are searching that residence as well, sources say.
Three people were brought to Danbury Hospital. Their conditions are not confirmed. The emergency room is on lockdown.
Vance said two children died at a local hospital and another had injuries.