Adam Lanza, a troubled 20-year-old loner with a history of autistic behavior, is the monster behind a horrific shooting at a Connecticut elementary school that left 26 people, including 20 children, dead on Friday.

Adam Lanza shot his mother Nancy, a kindergarten teacher, at the upscale suburban home they shared together and then took three of her guns and drove the Sandy Hook Elementary School about 9:30 a.m.

He used two semi-automatic pistols, a Glock and Sig Sauer, and reportedly wiped out an entire classroom of young children and shot several in a second class before taking his own life.

Witnesses say Adam Lanza was going from room-to-room shooting people after first killing the principal Dawn Hochsprung and psychologist execution-style after they confronted him in the hallway.

Parents who have not been united with their children are assuming the worst. One witness told WCBS: “Police just told us everyone presumed missing is in the school and they are dead.”

Students described being ushered from their classrooms hand-in-hand, with their eyes closed, to the safety of a nearby fire station as police converged on the school.

Adam Lanza’s brother Ryan, 24, who was originally thought to have been the shooter, is being questioned by police after he was arrested at his home in Hoboken, New Jersey.

Ryan Lanza was on a bus on his way home from work when he was being named as the gunman and posted on Facebook that it wasn’t him.

He told a friend he believed his mentally disabled brother did it. Adam Lanza is believed to be autistic.

Another relative told ABC Adam Lanza had been “obviously not well” and Beth Israel, tonight tweeted that her daughter went to school with him and he was “troubled for a long time”.

Alex Israel spoke to CNN about how she went to school with Adam Lanza and she described him as fidgety, and a quiet loner who kept to himself.

Adam Lanza is the monster behind a horrific shooting at a Connecticut elementary school that left 26 people dead on Friday

She also said he was a highly intelligent student, “above the rest of us”.

But she said he was never violent.

She tweeted earlier in the day that he was “crazy”.

The gunman’s father Peter Lanza, who is divorced from his wife, lives in Stamford, Connecticut, and declined to comment this evening after police performed a welfare check on him.

Neighbors said he recently remarried.

Peter Lanza learned of the tragic and senseless massacre by reporters who had flocked to his home.

He arrived shortly after police left and asked what the problem was.

The Stamford Connecticut said his expression shifted from patient to surprise to horror.

Newtown was once named one of the safest places to live in America and before this morning, there had been one murder in a decade.

The President addressed a stunned nation five hours after the shooting and openly wept as he spoke of the mindless shooting saying: “Our hearts are broken today.”

Seldom has a head of state expressed greater public emotion in modern times.

Barack Obama struggled for words, pausing several times as he wiped away tears saying: “This evening, Michelle and I will…hug our children a little tighter, and we’ll tell them that we love them.”

Adam Lanza is believed to have shot himself at the scene, according to the New York Times.

He was carrying three weapons including two handguns and a rifle and was wearing a bullet-proof vest, a mask and black military gear.

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. They were legally registered to Nancy Lanza.

A number of children were found hiding in closets in the school five hours after the shooting.

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