"They were standing. They were bound to each other. Their legs were bound so they couldn't run away," Miller said.
Roberts phoned the police, telling them that if the troopers didn't leave, he'd start shooting.
"Within a few seconds of that information being put out to the perimeter … they heard shots, multiple shots in quick succession," Miller said.
Police said the suspect was in the school for 45 minutes before they got the teacher's call.
Within nine minutes, police said they had surrounded the little school. Parents and community members came to the school as word spread across the lush green fields that something was wrong.
The hostage negotiator, who arrived with police, had no chance to talk with the suspect. Police said the arriving troopers "didn't have enough time to launch a full-scale assault" because the man began opening fire just as they were first trying to reach the gunman on his cell phone.
Police officers described what they found as "a horrific crime scene." At a news conference, Miller said that the suspect had barricaded himself inside with furniture and plywood that he had apparently brought along in his pickup truck for the purpose.
As ambulances and police cars gathered at the school in the bright midday sun, the usual country quiet was broken by news helicopters circling overhead.
Gary Kirchner, the Lancaster County coroner, told reporters it was "a terrible tragedy" in a community that considered itself, until today, virtually crime free.
The shooting dragged the Amish, who normally seem stuck in an idyllic, rural past of horse-drawn buggies and butter churns, straight into a national series of school attacks.
It was the third school attack in a week, after a student in Wisconsin shot and killed his principal and a drifter in Colorado took six female high school students hostage, molested them, and then, after shooting and killing one of them, shot himself.
Psychologists report that coverage of such events can have an unsettling effect on already disturbed individuals, inducing them to "copy" these crimes.
ABC News' Chris Cuomo, Bill Blakemore and Richard Esposito contributed to this report.