By RICHARD ESPOSITO, CANDACE SMITH (@CandaceSmith_) and CHRISTINA NG (@ChristinaNg27)
Dec. 14, 2012

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.

The massacre involved two gunmen at a Connecticut school this morning, prompting the town of Newtown to lock down all of its schools and draw SWAT teams to the school, authorities said today.

State Police confirm that one shooter is dead. A second gunman is apparently at large. Car-to-car searches are underway.

It's unclear how many people have been shot, but 25 people, mostly children are dead, multiple federal and state sources tell ABC News. That number could rise, officials said.

It is the worst shooting in a U.S. elementary school in recent memory.

The shooting comes just three days after masked gunman Jacob Roberts opened fire in a busy Portland, Ore., mall killing two before turning the gun on himself.

Today's shooting occurred at the Sandy Hook Elementary School in Newtown, about 12 miles east of Danbury.

Watch State Police News Conference Live at 1 p.m. at ABCNews.com

State Police received the first 911 call at 9:41a.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.
PHOTO: State police personnel led children from the school, following a shooting at Sandy Hook Elementary, Dec. 14, 2012.
Shannon Hicks/The Newtown Bee
State police personnel led children from the... View Full Size
Connecticut School Shooting: 3 Victims Hospitalized Watch Video
Connecticut School Shooting: 1 Gunman Confirmed Dead Watch Video
Florida School Shooting: Headmistress Dead Watch Video

A photo from the scene shows a line of distressed children being led out of the school.

LIVE UPDATES: Newtown, Conn., School Shooting

While some students have been reunited with their parents on the school's perimeter, one group of students remains unaccounted for, according to a source with a child in the school.

The school is kindergarten through fourth grade and has about 450 students.

CLICK HERE for more photos from the scene.

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 school 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."

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 mid-day bus runs.