UPDATE 5:10 p.m.: The Connecticut Post has created a detailed timeline of the shooting as it unfolded on social media. Nick Ivey tweeted, "heard the shots from a mile away."
UPDATE 3:55 p.m.: A Connecticut law enforcement official says 20 children were killed, along with six adults and the gunman. Eighteen children died in the school, two died in transit to the hospital.
UPDATE 3:40 p.m.: The New York Post is reporting that the alleged shooter was 20-year-old Adam Lanza, not 24-year-old Ryan Lanza, as was previously reported. Ryan Lanza is currently held by police, but according to the Post, is not considered a suspect.
UPDATE 3:20 p.m.: "Our hearts are broken today," said President Barack Obama, who spoke around 3:15 p.m. from the White House on the shooting. "As a country we have been through this too many times." Obama appeared to be fighting back tears as he spoke.
UPDATE 3:00 p.m.: A source has told the Associated Press that the gunman's younger brother is being held by authorities.
Speaker of the House John Boehner has ordered that all flags at the U.S. Capitol are to be lowered in tribute to the families and victims of the shooting.
UPDATE 2:45 p.m.: 1010 WINS Radio in New York City is reporting that Lanza apparently targeted his mother and her kindergarten classroom. CBS News is reporting that most of the shooting occurred in that one classroom.
UPDATE 2:35 p.m.: NBC News' Pete Williams says federal officials have identified the shooter as 24-year-old Ryan Lanza.
UPDATE 2:30 p.m.: Both CBS and NBC News are reporting that the shooter's mother, who was killed in the shooting, was a teacher at the school.
UPDATE 2:20 p.m.: NBC News' Pete Williams is reporting that a parent of the gunman was found dead in a home in New Jersey.
Hartfords’ WVIT-TV reported the principal of Sandy Hook Elementary was the target of the shooting. The News Times' Libor Jany, and WVIT are citing witnesses who are saying the school's principal was killed.
An official with knowledge of the details behind the Connecticut school shooting has told the Associated Press that 27 people are dead, including 18 children, after a gunman entered a school Friday and opened fire.
The shooting took place at around 9:30 a.m. at Sandy Hook Elementary School in Newtown, Connecticut, a town about 45 miles southwest of Hartford and 60 miles northeast of New York City.
The shooter was killed and apparently had two guns, a person with knowledge of the shooting told the AP. The person spoke on condition of anonymity because the investigation was still under way.
A law enforcement official says the attacker in the Connecticut school shootings is a 20-year-old man with ties to the school.
The official said that a gun used in the attacks is a .223-caliber rifle. The official also said that New Jersey state police are searching a location in that state in connection with the shootings.
WABC-TV in New York City is reporting that the shooter was armed four weapons and was sporting a bullet-proof vest.
Both CBS and NBC News report that a second person is in custody, but it is not yet known if the person was involved in the shooting.
According the Hartford Courant, one whole classroom is still unaccounted for.
A dispatcher at the Newtown Volunteer Ambulance Corps said a teacher had been shot in the foot and taken to Danbury Hospital. Andrea Rynn, a spokeswoman at the hospital, said it had three patients from the school but she did not have information on the extent or nature of their injuries.
The superintendent's office said the district had locked down schools in Newtown. Schools in neighboring towns also were locked down as a precaution.
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.
Connecticut's governor was on the scene in Newtown. The White House said President Barack Obama was notified of the shooting and was receiving updates.
This story will be updated as information becomes available.
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Greg Otto is a news editor at U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org.