At least 10 of the dead at Sandy Hook Elementary School were students, a different law enforcement source familiar with the investigation said.
"Why? Why?" one woman wailed as she walked up a wooded roadway leading from the school.
At a news conference Friday afternoon, state police Lt. Paul Vance declined to put a number on the deaths. He would say only that there had been "several fatalities." Police were waiting to notify families before releasing details, but he reassured nervous residents that the school was secure, the danger past.
The gunman is dead, his body in a classroom, a source with knowledge of the investigation told CNN. A law enforcement identified the suspected shooter as Ryan Lanza, who was in his 20s. Police have recovered two weapons from the suspect, a Glock and a Sig Sauer, the source said. It's unclear if police killed the suspect.
Police were questioning another man but are not calling him a suspect, a source with knowledge of the investigation told CNN.
Despite the danger being past, federal officers in tactical gear were on the scene Friday afternoon, coordinating with state and local authorities. At least four ambulances remained at the school.
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Hospital officials in neighboring Danbury said they were treating three people wounded in the shooting. Danbury Mayor Mark Boughton said the victims were in "very serious" condition.
Police and teachers rushed students from the building as police swarmed the area shortly after the first calls came in, around 9:40 a.m. ET. Officials moved students to a nearby firehouse, where parents frantically sought information about their children.
Third-grader Alexis Wasik said police and teachers barged into her classroom and told students to hide in the corner.
"Everybody was crying," she said. "And I just heard the police officers yelling."
Aerial images from CNN affiliate News 12 New Jersey showed police officers armed with rifles following a dog into woods near the school.
Other officers, some in tactical gear, stood around the school with guns drawn, CNN affiliate WFSB reported.
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All schools in the city were on lockdown Friday as police assessed the situation, Vance said.
Gov. Dan Malloy is "horrified" by what happened, a spokesman said. Malloy was meeting with families Friday afternoon, the spokesman said.
White House spokesman Jay Carney said the Obama administration feels "enormous sympathy" for affected families and pledged federal support for state and local law enforcement. Obama was expected to address the shooting on Friday.
If the number of fatalities holds up, the Newtown shooting would be the second-deadliest school shooting in U.S. history, behind only the 2007 shooting at Virginia Tech that left 32 people dead.
The 1999 Columbine High shooting in Littleton, Colorado, is the deadliest at a U.S. elementary, junior or high school. In that incident, two students shot 13 people to death before killing themselves.