A gunman opened fire in a one-room Amish schoolhouse in Lancaster County yesterday morning, killing at least four and injuring six, including three who are in critical condition at Penn State's Hershey Medical Center.

According to notes he left his family, the suspect, Charles Carl Roberts IV, 32, of Bart Township, was seeking revenge for an incident that took place 20 years ago before he took his own life, Pennsylvania State Police press secretary Jack Lewis said.

Lewis said as of 6 p.m. last night, three girls were dead at the crime scene and one died upon arrival at Lancaster General Hospital. He added that the other victims were transported to various hospitals.

Amy Stranges, team lead for communication at Penn State's Milton S. Hershey Medical Center, said the hospital received three victims yesterday afternoon.

Stranges said all of the girls -- ages 6, 8 and 13 -- are in critical condition with one gunshot wound each.

The two youngest victims were in pediatric intensive care and the 13-year-old was still in surgery as of 7:30 p.m. yesterday, Stranges said.

CNN reports described the schoolhouse situation as a "premeditated hostage scenario."

Roberts seemed to have killed the students, ages 6 to 13, "execution style" as they were lined up against the blackboard, State Police Commissioner Jeffrey Miller said during a CNN televised press conference yesterday afternoon.

Lewis said the gunman entered the school at about 10 a.m. and let 15 male students, the teacher, teacher aides and adults with infants leave the premises. He then proceeded to barricade the schoolhouse with two-by-four and two-by-six boards and bound the remaining females with wire ties and flex-cuffs, Lewis said.

Miller called the incident "a horrendous, horrific incident for the Amish community."

Police said yesterday they had found no connection between this incident and the recent school shooting in Colorado, in which all males were also released.

Lewis said police responded to the Lancaster situation immediately after the teacher managed to call 911 -- about 45 minutes after she was released from the schoolhouse, because there was no phone in the vicinity.

Roberts then called the police and demanded troopers abandon the area within 10 seconds or he would open fire, Lewis said.

Lewis said troopers attempted mediation with the suspect after receiving his cellular phone number from the 911 operators.

Within seconds of dialing the number, "shots rang out" at the schoolhouse, Lewis said.

He said about 10 troopers attempted to rush the building as Roberts used a handgun to shoot the girls. He also took shots at the troopers, who Lewis said met resistance by Roberts' secured barricades blocking the entrances.

Lewis said gunshots soon stopped, and police managed to enter the building through a window.

"There was silence," Lewis said. "[Troopers] looked in and saw [Roberts] on the floor dead."

Roberts worked as a truck driver for a milk company in the Lancaster area. Lewis said it was likely Roberts serviced the area surrounding the school and possibly knew the children.

His motives are still being investigated.

Eric Stauffer (sophomore-physical education and kinesiology), a Lancaster native, said he heard about the situation from a friend and they watched the televised reports together.

He added that he's known a couple of Amish while working at a Lancaster field mill and a farm.

"It's just really interesting because anyone that knows the Amish knows they're not really resistant. You could punch a guy in the face and he wouldn't do anything back," he said. "The fact that [Roberts] walked into an Amish school is really weird."

Kyle Horst (sophomore-architectural engineering) said he lives about 10 miles from the area of the crime and his mother works at a doctor's office about a half mile from the schoolhouse in Paradise County.

He said the doctor's office is primarily Amish, and he knows a lot of people in the town.

"I was shocked when I found out," he said. "Kind of scary to think he did that."