10:03 p.m.: WABC News and ABC News report that the Sandy Hook elementary school principal, Dawn Hochsprung, was among those killed during the Newtown, Conn., school rampage, according to multiple sources.

9:37 p.m.: A Sig Sauer and a Glock semi automatic handgun were used in the slaying and that .223 shell casings - a round used in a semi automatic military style rifle - were also found.

9:16 p.m.: Numerous relatives and friends of the gunman, Adam Lanza, and his mother were being interviewed by the FBI in New Jersey, Connecticut and Massachusetts, all in an effort to put together a better picture of the gunman and any possible explanation for today's tragedy.

7:53 p.m.: Connecticut State Police Lt. J. Paul Vance said at a news conference that would be no more pressers tonight and they will not be releasing the identities of the victims tonight. "We've never seen anything like this. It's heart wrenching for us," Vance said.

7:30 p.m.: Families of Virginia Tech victims release statement on Sandy Hook School shooting: "The immediate concern we have is for those families in Newtown whose lives are now altered, forever. From our VTV experience, as families of the victims and survivors of the April 16, 2007 massacre at Virginia Tech, we know that their loss is incredible. Helping all to deal with the aftermath will take a lifetime of love and dedication," Joe Samaha, President of VTV Family Outreach Foundation, said in a statement.

7:00 p.m.: Authorities continue to question Ryan Lanza, the shooter's brother, but say he is not under any suspicion.

Tune in to World News with Diane Sawyer at 6:30 p.m. ET.

6:25 p.m.: Nancy Lanza, mother of gunman Adam Lanza, was found dead in her Connecticut home. The 20-year-old man, shot her in the face, authorities told ABC News. Following the murder of his mother, Lanza, carrying at least two semi-automatic pistols and an automatic rifle, drove to Sandy Hook Elementary School in Newtown, Conn., where his mother worked. There he killed 20 children and six adults. Lanza was found dead in the school with a self-inflicted gunshot wound. The rifle was found in his car, authorities said.

Adam Lanza, seen here in 2005, killed his mother at her Connecticut home before killing 20 elementary school students and 6 adults at Sandy Hook Elementary School in Newtown, Conn, authorities said.

6:00 p.m.: "Evil visited this community today," said Conn. Gov Dan Malloy.

5:29 p.m.: Gunman's mother was teacher's aide at school and not a teacher as previously believed, authorities say. WATCH LIVE press conference with Conn. State Police

4:52 p.m: Neighbors described Adam Lanza to ABC as "odd" and displaying characteristics associated with mental illness.

4:41 p.m.:Federal law enforcement sources now say Connecticut school shooting gunman was Adam Lanza, 20, and not his brother Ryan Lanza, 24, as earlier reported. Adam was found dead in the school along with his mother, a kindergarten teacher there.

President Obama earlier today ordered flags flown at half staff at all federal and military buildings and installations. Of those thousands of flags, perhaps none flew as mournfully as this one in the town center of Newtown, Conn., hours after 18 children were killed in school shooting. (Kyle Lyddy)

4:18 p.m.: "My deepest sympathies are with families of those affected, and my determination to stop this madness is stronger than ever," tweets New York City Mayor Michael Bloomberg, a staunch gun-control advocate.

4:17 p.m.: Federal sources initially identified the suspect as Ryan Lanza, 24, of New Jersey, but there appears now to be confusion over the gunman's identity.

3:42 p.m.: Shooting took place in "one section of the school in two rooms," says Vance, adds there is only one known shooter "right now."

3:40 p.m.: 18 children pronounced dead at school; 2 dead at hospital; 6 adults dead at school, says Conn. State Police spokesman Paul Vance. WATCH LIVE press conference with Conn. Gov. Dan Malloy.

3:18 p.m.: "As a country, we have been through this too many times," President Obama says about shooting.

3:17 p.m.: President Obama says majority of victims were students ages five to 10 years old. READ TRANSCRIPT of President Obama's remarks on Connecticut school massacre.

3:15 p.m.:President Obama visibly emotional in remarks about Newtown, Conn. school shooting.

A clearly moved President Obama tells the nations most of the victims killed in today's Newtown, Conn. elementary school shooting were children aged 5 to 10.

WATCH LIVE President Obama's remarks on school shooting.

3:15: Gunman Ryan Lanza's mother was a kindergarten teacher at the school. She and many of her students were found dead in the school.

2:48 p.m.: Hero teacher Kaitlin Roig tells ABC News that she barricaded her first grade students in classroom bathroom and locked the door when she first heard gun shots. "The kids were being so good. They asked: 'Can we go see if anyone is out there? … I just want Christmas… I don't want to die, I just want to have Christmas.' I said, you're going to have Christmas and Hanukkah.. I tried to be positive."

2:46 p.m.: White House announces that President Obama will address the nation at 3:15 p.m. Watch it HERE.

2:37 p.m.: Gunman Ryan Lanza's mother found among the dead at Sandy Hook Elementary School.

2:28 p.m. The gunman in the Newtown, Conn., school shooting has been identified as Ryan Lanza, 24, of New Jersey. A dead body has also been found in his parents' home, officials said. Lanza's body was found in the school.

2:22 p.m.: Federal agents investigating whether slain Connecticut school gunman had a relationship with someone in the school, sources say.

1:44 p.m.: "The scene is secure. The public is not in danger," says Connecticut State Police spokesman Paul Vance.

1:42 p.m.:" Shooter is deceased inside the [school,]" says Connecticut State Police spokesman Paul Vance. WATCH LIVE press conference on school shooting.

Parents leave a staging area after being reunited with their children following a shooting at the Sandy Hook Elementary School in Newtown, Conn. (Jessica Hill/AP)

1:26 p.m.: President Obama has been briefed on shooting by FBI director Bob Mueller and spoken with Conn Gov. Dan Malloy.

1:17 p.m: One shooter is described as 24-years-old, armed with four weapons and wearing a bullet-proof vest, sources tell ABC News.

1:09 p.m.: Twenty-seven people, or more, most of them children killed in shooting, sources tell ABC News.

12:57 p.m.: Fatality numbers continue to rise. More than 25 people, mostly young children, killed in Newtown, Conn. school shooting, sources tell ABC News. WATCH live coverage of press conference at scene of school shooting, beginning at 1p .m.

12:47 p.m.: More than 20 people, most of them young children, killed in elementary school shooting, according to law enforcement sources.

12:36 p.m.: Law enforcement conducting car to car searches in area of school. Eyewitness PHOTOS from scene of school shooting.

12:26 p.m.: More than a dozen persons, including children shot and killed at Newtown, Conn., elementary school, federal, and state law enforcement sources tell ABC News.

12:24 p.m.: Law enforcement agents with weapons drawn seen running into wooded area behind the school.

12:19 p.m.:Federal agents are at the scene of the school with guns still drawn as parents rush to pick up their children.

State police personnel led children from the school, following a shooting at Sandy Hook Elementary.

12:11 p.m: LIVE streaming video from chopper above Sandy Hook Elementary.

12:09 p.m.: Sources tell ABC News multiple persons shot

11:55 a.m.: Danbury Mayor Mark Boughton tells WABC three victims in hospital are in "very serious condition." 11:52 a.m.: Danbury Hospital confirms three patients have been taken there for treatment. A shooting involving two gunmen erupted at a Connecticut elementary 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, sources told ABC News. The shooting occurred at the Sandy Hook Elementary School in Newtown, 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.