More than two dozen people, mostly elementary school children, were shot and killed at a Newtown, Conn., elementary school this morning, federal and state sources tell ABC News.
Authorities initially believed that there were two gunmen and were performing car-to-car searches around the school. It is currently unclear whether there is still a suspect at large.
"The shooter is deceased inside the building," Connecticut State Police spokesman Lt. Paul Vance said at a news conference. "The public is not in danger."
Authorities said the school building is now secure.
One shooter was described as a 24-year-old armed man with four weapons and wearing a bullet-proof vest, sources told ABC News.
The massacre prompted the town of Newtown to lock down all its schools and draw SWAT teams to the school, authorities said today.
It's unclear how many people have been shot, but 27 people, mostly children, are dead, multiple federal and state sources tell ABC News. That number could rise, officials said.
President Obama was briefed on the shooting by FBI Director Robert Mueller.
During a call with Connecticut Gov. Dan Malloy, authorities at the news conference said Obama called to "express his condolences and condolences on behalf of the nation and pledge whatever resources the federal government can bring to bear to assist the families and to assist the investigation."
It is the worst shooting in a U.S. elementary school in recent memory and exceeds the carnage at the 1999 Columbine High School shooting in which 13 died and 24 were injured. It is the second worst mass shooting in the country's history after the Virginia Tech shooting in which 32 people were killed before the shooter turned the gun on himself.
The Newtown shooting comes three days after masked gunman Jacob Roberts opened fire in a busy Oregon mall, killing two before turning the gun on himself.
Today's shooting occurred at the Sandy Hook Elementary School, which includes 450 students in grades K-4. The town is located about 12 miles east of Danbury.
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State Police received the first 911 call at 9:41 a.m. and immediately began sending emergency units from the western part of the state. Initial 911 calls stated that multiple students were trapped in a classroom, possibly with a gunman, according to a Connecticut State Police source.
A photo from the scene shows a line of distressed children being led out of the school.
Three patients have been taken to Danbury Hospital, which is also on lockdown, according to the hospital's Facebook page.
"Out of abundance of caution and not because of any direct threat Danbury Hospital is under lockdown," the statement said. "This allows us simply to focus on the important work at hand."
Newtown Public School District secretary of superintendent Kathy June said in a statement that the district's schools were locked down because of the report of a shooting. "The district is taking preventive measures by putting all schools in lockdown until we ensure the safety of all students and staff," she said.
State police sent SWAT team units to Newtown.
All public and private schools in the town are on lockdown.
"We have increased our police presence at all Danbury Public Schools due to the events in Newtown. Pray for the victims," Newtown Mayor Boughton tweeted.
State emergency management officials said ambulances and other units were also en route and staging near the school.
A message on the school district website says that all afternoon kindergarten is cancelled today and there will be no midday bus runs.