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 and prompted the town of Newtown to lock down all of its schools and draw SWAT teams to the school, authorities said today. One shooter is dead and a manhunt is on for a second gunman. Police are searching cars. One shooter was described as a 24-year-old armed with four weapons and wearing a bullet vest, sources told ABC News. 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. President Obama was briefed on the shooting by FBI Director Bob Mueller. It is the worst shooting in a U.S. elementary school in recent memory and exceeds the carnage at 1999 Coumbine High School shooting in which 13 died and 24 were injured. The Newtown shooting comes just 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 from kindergarten through fourth grade. The town is located about 12 miles east of Danbury. Watch Upcoming State Police News Conference Live 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. A photo from the scene shows a line of distressed children being led out of the school. LIVE UPDATES: Newtown, Conn., School Shooting 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. 27 People Dead, Mostly Children, at Connecticut Elementary School Shooting - Yahoo!