Update 3:25 p.m.:
Adam Lanza's older brother, Ryan, 24, of Hoboken, N.J., is being questioned by police, said the first official. Earlier, a law enforcement official mistakenly transposed the brothers' first names.
Both officials spoke on the condition of anonymity because they were not authorized to speak on the record about the developing criminal investigation.
The first official said Adam Lanza is dead from a self-inflicted gunshot wound.
According to the second official, the suspect drove to the scene of the shootings in his mother's car. Three guns were found at the scene — a Glock and a Sig Sauer, both pistols — and a .223-caliber rifle. The rifle was recovered from the back of a car at the school. The two pistols were recovered from inside the school.
The official also said Lanza's girlfriend and another friend are missing in New Jersey.
Meanwhile, former Jersey Journal staff writer Brett Wilshe said he has spoken with Ryan Lanza of Hoboken, who told Wilshe the shooter may have had Ryan Lanza's identification.
Ryan Lanza has a Facebook page that posted updates Friday afternoon that read that "it wasn't me" and "I was at work."
Associated Press writers Adam Goldman in Washington and Samantha Henry in Newark, N.J., contributed to this report.
Update 3:00 p.m.:
Police say 27 people were killed in the shooting at a Connecticut elementary school, including the gunman, and one person died at another scene.
The dead at the school include 20 children.
The gunman opened fire Friday morning inside a school where his mother worked. He blasted his way through the building as young students cowered helplessly in classrooms while their teachers and classmates were shot.
The gunman killed himself. Gov. Dannel P. Malloy says someone who lived with him also died.
Young students crying and looking frightened were escorted by adults through a parking lot in a line after the shots rang out in Newtown, 60 miles northeast of New York City.
The attack comes less than two weeks before Christmas and appears to be the nation's second-deadliest school shooting, exceeded only by the Virginia Tech massacre in 2007.
Update 2:40 p.m.:
Conn. state police: 26 dead, including 18 children, at school; 2 later at hospital; gunman also dead.
Update 2:25 p.m.:
NEWTOWN, Conn. (AP) — More information is emerging about the suspect in today's Connecticut elementary school shootings. A law enforcement official identifies him as 24-year-old Ryan Lanza.
Another official who was briefed on the shooting says the gunman died of a self-inflicted gunshot wound -- and that one of the victims was the shooter's mother, a teacher at the school.
According to one official, the suspect's younger brother is being held for questioning as a possible second shooter.
The official also says Ryan Lanza's girlfriend and another friend are missing in New Jersey.
Although authorities haven't released the number of dead and injured, an official says 26 people were killed, including 18 children, in addition to the gunman. That would make the shooting the nation's second-deadliest at a school -- exceeded only by the Virginia tech massacre in 2007.
Students are telling parents about their ordeal this morning. One 6-year-old has told his father that he was in a classroom when a gunman burst in and shot the teacher. The boy said he grabbed some of his friends and ran from the room.
President Obama Makes a Statement on the Shooting in Newtown, Connecticut
Update 1:56 p.m.:
President Barack Obama will speak from the White House briefing room about the mass shooting at a Connecticut elementary school.
The president's comments will be at 3:15 EST Friday. They will be his first on the incident, which has left at least 27 people dead, including 18 children.
White House spokesman Jay Carney said that mass shootings "weigh heavily" on Obama as the president and as a father. Carney said that Friday was not the day to talk about gun control, however.
Obama was not expected to take questions from the media.
Update 1:00 p.m.:
State police: Students and staff among victims at Conn. elementary school, shooter dead.
Update 12:30 p.m.:
NEWTOWN, Conn. (AP) — There's been a deadly shooting at an elementary school in Connecticut — where an official says 27 people have died, including 18 children.
Another official says the gunman at Sandy Hook Elementary in Newtown was killed. The official says the shooter apparently had two guns.
Frantic parents rushed to the school to check on their children. One man says his 8-year-old daughter heard two big bangs, and teachers told her to get in a corner. The daughter was fine.
A woman says it was "the happiest moment" of her life when she found her own 8-year-old daughter safe.
The 17-year-old brother of a 9-year-old girl at the school says he raced to the school, and found she was OK. He says his sister heard a scream come over the intercom at one point. The teen says teachers were shaking and crying as they came out of the building.
A photo in the Newtown Bee newspaper showed a group of young students being escorted through a parking lot by adults. Some were crying, and others looked visibly frightened.
The school superintendent's office in Newtown, about 60 miles northeast of New York City, says the district locked down all of the schools in the town. Schools in neighboring communities were also locked down.
The White House says President Barack Obama was notified of the shooting.
Update from 12:00 p.m.:
The official spoke on condition of anonymity because the investigation is still under way.
Another official, speaking on condition of anonymity for the same reason, says the gunman is among the dead.
The killings happened at Sandy Hook Elementary School in Newtown, about 60 miles northeast of New York City.
State police say Newtown police called them around 9:40 a.m. A SWAT team was among the throngs of police to respond.
Photos from the scene showed young students — some crying, others looking visibly frightened — being escorted by adults through a parking lot in a line, hands on each other's shoulders.