Then, 15 minutes later, as he was heading from his house near Sandy Hook Elementary school to a diner, he saw the children.
There were six of them, small children sitting in a neat semicircle at the end of his driveway. A school bus driver was standing over them, telling them things would be all right. It was about 9:30 a.m., and the children, he discovered, had just run from their school to escape a gunman.
"We can't go back to school," one little boy told Rosen. "Our teacher is dead. Mrs. Soto; we don't have a teacher."
That's when Rosen's grandfatherly instincts kicked in.
The 69-year-old retired psychologist took the four girls and two boys into his home, and over the next few hours gave them toys, listened to their stories and called their frantic parents.
"I had no idea what had happened," Rosen said. "I couldn't take that in."
Victoria Soto, 27, was a first-grade teacher killed when 20-year-old Adam Lanza burst into her classroom. It wasn't clear how the children escaped harm, but there have been reports that Soto hid some of her students from the approaching gunman. The six who turned up at Rosen's home did apparently have to run past her body to safety.
"They said he had a big gun and a little gun," said Rosen, who didn't want to discuss other details the children shared.
On Friday, Rosen walked the children past his small goldfish pond with its running waterfall, and the garden he made with his two grandchildren, into the small yellow house he shares with his wife.
He ran upstairs and grabbed an armful of stuffed animals. He gave those to the children, along with some fruit juice, and sat with them as the two boys described seeing their teacher being shot.
Then Rosen called the children's parents, using cellphone numbers obtained from the school bus company, and they came and retrieved their children.
One little girl, he said, spent the entire ordeal clutching a small stuffed Dalmatian to her chest and staring out the window looking for her mommy.
And one little boy brought them all a moment of levity.
"This little boy turns around, and composes himself, and he looks at me like he had just removed himself from the carnage and he says, 'Just saying, your house is very small,'" Rosen said. "I wanted to tell him, 'I love you. I love you.'" Rosen said Sandy Hook had always been a place of joy for him. He taught his 8-year-old grandson to ride his bike in the school parking lot and took his 4-year-old granddaughter to use the swings.
"I thought today how life has changed, how that ground has been marred, how that school has been desecrated," he said.
A couple of hours after the last child left, a knock came on his door. It was a frantic mother who had heard that some children had taken refuge there. She was looking for her little boy.
"Her face looked frozen in terror," Rosen said, breaking down in tears.
"She thought maybe a miracle from God would have the child at my house," he said. Later, "I looked at the casualty list ... and his name was on it."
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