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Quickly, quietly she and other library staff got the 18 children down on the floor and crawled with them to a classroom storage closet. Hiding from the gunman who killed 20 children and six adults at Sandy Hook Elementary School, they barricaded themselves inside by shoving a file cabinet against the door.
"We settled them down with paper and crayons," Jacob told reporters on Saturday.
The gunfire suddenly ended and police came pounding at the door. But the library staff refused to open it until they slipped a badge under the door, Jacob said.
In the aftermath of the massacre, Connecticut Governor Dannel Malloy said "evil" had descended upon the small community of Newtown. But emerging a day after the carnage were tales of heroism by school staff members, including the six who died.
There was first-grade teacher Vicki Leigh Soto, 27, who police said "put herself between the kids and the gunman's bullets" and whose body was found huddled with the students in a classroom closet, according to The Wall Street Journal.
And there were selfless survivors like first-grade teacher Kaitlin Roig. She told ABC News she scrambled her class into a cramped bathroom, locked the door and "told the kids I love them" in case those were the last words they ever heard.
A school custodian reportedly raced through the hallways echoing with gunfire to check that classroom doors were locked from the inside, the Newtown Bee newspaper said.
On Friday morning fourth graders were in Jacob's library classroom when the intercom sputtered to life with what sounded like a struggle in the school office.
"We heard some scuffling noises and stuff and I thought someone made a mistake," Jacob said. "So I called down there and the secretary answered the phone and said 'There's a shooting.'"
Then Jacob heard "popping noises" that she realized was gunfire.
"I shouted 'Lockdown!' and I ran across the hall and told the other class it was a lockdown," Jacob said.
She dashed back to her classroom and discovered that the door would not lock. Spying the storage closet in the room, Jacob and the rest of the library staff guided the children in a group-crawl to safety as the gunfire continued.
"We tried to minimise it with the kids. Just tried to keep it calm and quiet," she said. The staff told the children it was an active shooter drill that they had practiced before.
Later, Jacob said she found out that "the kids who died were in two first-grade classrooms."
Panicked parents converged on a firehouse near the school on Friday afternoon, terrified by the thought that their children might be among the dead.
"The teachers lined up, held up signs, the kids lined up behind them," Jacob said.
"There were a lot of parents running around. It came out pretty quickly that there were almost two full classes missing," she said.