The son of a Connecticut kindergarten teacher stormed her Newtown school this morning and fatally shot 20 children and six adults in the worst elementary school shooting in American history.

The shooting was unimaginable and stunned the nation. In Newtown, hundreds of people gathered into the night to brace for the painful days ahead.

Upon entering Sandy Hook Elementary School, the gunman, identified as 20-year-old Adam Lanza, opened fire first in the school's office, then in a kindergarten class, authorities told NBC News. He was found dead at the scene. A woman believed to be his mother was found fatally shot in a Newtown home.

“Evil visited this community today,” Gov. Dannel Malloy said. "And it's too early to speak of recovery, but each parent, each sibling, each member of the family has to understand that Connecticut -- we are all in this together."

President Obama teared up as he spoke of the tragedy. "The majority of those who died today were children. Beautiful little kids between the ages of 5 and 10 years old," he said at a news conference. "They had their entire lives ahead of them. Birthdays, graduations, weddings, kids of their own.

"So our hearts are broken for them today."

The massacre at Sandy Hook Elementary School is one of the worst school shootings in American history, second only to the 2007 Virginia Tech shooting that killed 32.

The shooting took place during the morning annoucements, students said. Dozens of state police troopers converged at the school shortly after the 911 call came in, but it was too late: Emergency vehicles took only three victims to Danbury Hospital; 24 lay dead in the school. Two of the victims died at the hospital.

Students and parents described a scene of terror when a black-clad gunman stormed the school. He was armed with two 9 mm pistols, legally purchased and registered to his mother, authorities later said.

"I was in the gym and I heard a loud, like seven loud booms, and the gym teachers told us to go in the corner, so we all huddled," one student told NBC Connecticut. "And I kept hearing these booming noises. And we all … started crying.

"All the gym teachers told us to go into the office where no one could find us," she added. "So then a police officer came in and told us to run outside. So we did and we came in the firehouse and waited for our parents."

The brother of one student said he went to the school to get his little sister and watched as students were being brought out of the building.

His sister was crying and said she heard screams coming over the intercom.

“When they were leaving the room … (state police) were telling the students to hold hands and close their eyes until they were outside,” he said.

For hours, information was sparse and it was not even clear anyone had been gravely injured until after noon.

The shooter's brother, 24-year-old Ryan, of Hoboken, N.J., was initially believed to be the gunman, law enforcement officials said. But they adjusted their report after they found Ryan at his home in Hoboken, N.J., NBC News reported. He is cooperating with investigators.

By Friday evening, hundreds of people had packed into St. Rose of Lima Roman Catholic Church in Newtown for a standing-room-only servie and vigil. Dozens more watched through open windows and doorways, praying and weeping and holding their kids tights.

Monsignor Robert Weiss told the crowd that he'd spent five hours with the victims' families, trying to console them and asking questions that could not be answered. The attack defied logic and explanation, he said.

"This is Newtown, a place where parents bring their children to be safe," he said.

"I think about those children ... and I know we have 20 new saints, 20 new angels," he said, his voice halting from grief. "But that doesn't take away the pain. It doesn't take away the hurt. But it does bring us closer together."

Outside the church mourners lit candles and left flowers at the feet of a statue of Ave Maria near an inscription that read: "This shrine is dedicated to loving parents by their childrens and families."

As the investigation continues, officers have been assigned to the parents so the information is communicated directly to them, according to state police.

On Saturday, the FBI crisis management team will be in Newtown to provide assistance as work on the investigation continues.

“Earlier today a tragedy of unspeakable terms played itself out in this community,” said Malloy, who met with the families of victims. “You can never be prepared for this kind of incident."