Authorities are sorting through the facts of why a gunman forced his way into an elementary school and opened fire on defenseless children in what has been called the second-most deadly school shooting in the U.S., after the 2007 shooting at Virginia Tech massacre that took 32 lives.

New details are emerging that paint how the Friday morning events unfolded and exactly who the gunman was. But it may take days, a police spokesman said, to know the full motive.

What is known is this: December 14 at Sandy Hook Elementary school in Newtown, Connecticut, started out as any other -- until 20-year-old Adam Lanza killed 27 people, including 20 children, six adults, plus his own mother. He then shot and killed himself.

Around 9:30 a.m., while classes were underway for about 30 minutes, Lanza approached the school's front entrance and forced his way in. Broken windows, which also could have been smashed by police, were observed at the school.

Dressed in black battle fatigues and a military vest, the gunman began firing shots in a hallway, and principal Dawn Hochsprungshot, the vice principal and school psychologist left the office and followed the sound of gunfire to the hallway, where they were shot. The vice principal survived her injuries, but the principal and psychologist passed away. More than 600 children -- from Kindergarten to 4th grade -- were inside the school.

The shooter then went to two classrooms, the first of which was a Kindergarten room, where his mother, Nancy Lanza, was believed to have either worked as a teacher's aid or as a volunteer. Then he moved to a next-door classroom and again opened fire. All of the victims, the medical examiner said today, were shot multiple times.

Teachers in the rest of the building did what they could to protect their students, pulled some children in from cooridors, locking their doors and hiding children in corners and closets. According to ABC News, one teacher hid her students in a bathroom.

A school employee called 911 and within minutes, officers arrived on the scene, taking control of the building, securing rooms, and making sure it was one gunman and that they knew where he was. Children told reporters they could hear police officers on the roof.

A custodian ran through the hallways, warning of a gunman. Someone switched on the intercom, a move which could have saved many lives by letting them hear the chaos in the hallway.

All told, from the time the shooter walked in to the time police had the school secured, it took about 20 minutes.

Family friend Barbara Frey told ABC News that Adam Lanza, who played with her son when they were growing up, "was not connected with the other kids." Lanza, an honor student and honors graduate of Newtown High School, was not known to have any friends. Police said he did not have a criminal record. Some people who knew him reportedly said the 20-year-old gunman snapped and went on a rampage.

But Mary Ellen O'Toole, an FBI profiler, said otherwise. "He did not just snap this morning and decide to commit violence," she told aBC News. "it takes time to plan. What appears to be snap behavior isn't that at all."

Don White, who was with the Bexar County Sheriff’s Department’s homicide investigation division in San Antonio, Texas, agreed. He told me in an interview that what Lanza carried out took time to plan.

Three weapons recovered by officials at the scene have been reported as two 9mm handguns, a Glock and a Sig Sauer, and a .223-caliber Bushmaster rifle, used globally by military and law enforcemen, all found near Lanza's body. It was reported that the shooter fired as many as 100 rounds during the shooting spree. A fourth weapon, a Saiga shotgun, was found in the trunk of the car.

The guns, police said, were registered and purchased by Adam Lanza's slain mother, who has been referred to as a gun enthusiast and who went to shooting ranges with her sons. Nancy Lanza was found dead, from a gunshot wound to her face, inside her Newtown home where her son Adam lived with her.

HLN reported that the gunman tried to buy a gun in the area three days before the shooting but was turned away because he was underage.

His older brother Ryan, who was initially misidentified by police as the shooter because the gunman had been carrying his brother's ID, described his brother to police as having a "personality disorder." He also told authorities he had not seen his brother since 2010. The brothers' father, Peter Lanza, who had been divorced from their mother for about three years, voluntarily was interviewed by police and the FBI, along with Ryan. They are not connected to the shooting.

Lt. J. Paul Vance, a spokesman for the Connecticut State Police, said during a news conference, "We are specifically investigating the shooter." On Saturday, Lance said during a news conference, "We are learning new facts about how and, most importantly, why this happened."

What they know thus far is that Lanza had argued with his mother that morning, shot and killed her and then used her car to drive to the school.

"He killed his mom, went to school, probably knowing that they would buzz him inside the locked door," White said, "went to the office and assaulted them, then to the classroom, possibly knowing the substitute teacher wouldn't go into lock-down mode after hearing the shots in the office. There is no way the teacher wouldn't have heard the shots that were fired in the office. The kids, being small, would be gravely hurt by almost any of those weapons," White said.

A law enforcement official, speaking on condition of anonymity, told FOX News that investigators believe that Lanza attended the school several years ago but appeared to have no recent connection to it.

Authorities have identified all of the victims killed in the tragedy but are not releasing a formal list of names and birthdates until the state medical examiner has completed his work, said Lt. Vance said.

President Obama, in his weekly address to the nation, echoed what he had said the day before immediately after the shooting: "Any of these neighborhoods could be our own. So we have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics."

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