[Kimberley]

(CBS/AP) At least four people were shot to death at a one-room Amish schoolhouse in Pennsylvania's bucolic Lancaster County Monday, and at least three young girls were hospitalized in critical condition, authorities said.

A total of seven students were hospitalized with injuries.

It was the nation's third deadly school shooting in less than a week, and one of the deadliest in U.S. history.

Police say the gunman who attacked an Amish school was 32 years old and sent the male students outside while keeping the girls in and blocking the doors. The number of those killed or injured is unclear.

"So far, six confirmed dead, and the helicopters are pulling into (Lancaster General Hospital) like crazy," Coroner G. Gary Kirchner said earlier.

State Police Cpl. Ralph Striebig said earlier that the shooter was dead.

The adult male suspect shot himself after lining up students, tying females' ankles and shooting some execution style in the head.

Three girls, ages 6-12, were admitted to Lancaster General Hospital in critical condition with gunshot wounds, spokesman John Lines said. Officials at the Penn State Milton S. Hershey Medical Center confirmed that victims also were being admitted there.

WGAL-TV reported that a man entered the schoolhouse this morning and began to make threats. The man apparently took hostages and began negotiations with the police. At some point, at least 10 shots were fired within the school. Police said the hostage taker shot himself, according to the TV station.

"They're all little kids," a man who lives near the school told WGAL. "I see the kids come right by my house … they're always friendly, waving, smile. They come to our house sometimes selling different goods … They're sweet kids. It just blows my mind. It's as quiet as can be out here."

Police late Monday morning surrounded the one-room schoolhouse, a tiny building surrounded by a white fence and farm fields in southeastern Lancaster County. The Lancaster County 911 Web site reported that dozens of emergency units were dispatched to a "medical emergency" at 10:45 a.m.

Hours after the attack, about three dozen people in traditional Amish clothing, broad-brimmed hats and bonnets stood nearby speaking to one another and authorities. Others gathered with a group of children at a nearby farm while investigators stretched out in a line across a field searching for evidence.

The school is just outside Nickel Mines, a tiny village about 55 miles west of Philadelphia.

It was the third small community to be shocked by a deadly school shootings in just the past week.

On Friday, a school principal was gunned down in Cazenovia, Wis. A 15-year-old student, described as upset over a reprimand, was charged with murder in the killing. Just two days earlier, an adult gunman held six girls hostage in a school at Bailey, Colo., before killing a 16-year-old girl and then himself.

Nationwide, the 1999 Columbine High School massacre in Littleton, Colo., remains the deadliest school shooting, claiming the lives of 15 people, including the two teenage gunmen. Last year, a 16-year-old shot seven people to death at a school on Minnesota's Red Lake Indian Reservation, then killed himself.