More than two dozen people, mostly children, were shot and killed at a Newtown, Conn., elementary school this morning by a heavily armed man who was killed inside the building, federal and state sources tell ABC News.

"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."

The gunman has been identified as Ryan Lanza, 24, of New Jersey. A dead body has also been found in his home, officials said. Sources said Lanza was armed with four weapons and wearing a bullet-proof vest when he opened fire in the elementary school.

Among the dead was the gunman's mother, found in the school, sources told ABC News.

LIVE UPDATES: Newtown, Conn., School Shooting

Authorities initially believed that there were two gunmen and were searching cars around the school. It is currently unclear whether there is still a suspect at large.

The massacre prompted the town of Newtown to lock down all its schools and draw SWAT teams to the school, authorities said today.

President Obama was briefed on the shooting by FBI Director Robert Mueller.

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.

CLICK HERE for more photos from the scene.

It is the second worst mass shooting in U.S. history, exceeded only by the Virginia Tech shooting in 2007 when 32 were killed before the shooter turned the gun on himself. Today's carnage exceeds the 1999 Columbine High School shooting in which 13 died and 24 were injured.

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.

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.

Vance said that on-duty and off-duty officers swarmed to the school and quickly checked "every door, every crack, every crevice" in the building looking for the gunman and evacuating children.

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 were 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.

Good thing we have the death penalty... It would have been a pain trying to catch this guy alive. Much less KEEP him alive for the rest of his life.