Updated

A bomb threat shut down an entire school district Thursday, canceling classes for more than 7,200 students in eight public schools, as well as hundreds more in area religious schools and child care centers.

Culpeper Sheriff H. Lee Hart said the threat, which came on the heels of three deadly U.S. school attacks in the span of a week, was called into the county's communications center at about 11:30 p.m. Wednesday.

"The individual had a lengthy conversation, and in this conversation he said he was going to blow up schools," Hart said. "We do not know which schools or his plans. We don't know a lot about this individual."

State police used explosives-detecting dogs and bomb technicians in a search of the buildings on Thursday, and the Bureau of Alcohol, Tobacco and Firearms was called for assistance, officials said. Experts planned to sweep all the public school buildings and several private schools for bombs or other possible dangers.

"You simply can't be too cautious," said Culpeper County Schools Superintendent David A. Cox. "The safety of our students and our employees is our number one concern."

Culpeper, about 60 miles southwest of Washington, D.C., is in a mostly rural area that has become a bedroom community for people who work in the Capitol area.

In the past two weeks, three schools in three states have been hit by deadly attacks, and several others have faced threats.

A gunman killed himself and five girls Monday at a one-room Amish schoolhouse in Pennsylvania; on Friday a 15-year-old Wisconsin student was arrested in the shooting death of his principal; and on Sept. 27 a man took six girls hostage in Colorado, sexually assaulting them before fatally shooting one girl and killing himself.

In addition, schools in states including Nebraska, Nevada, Oklahoma, Oregon and Wisconsin have been closed or locked down in the past week over threats of violence or guns on campus.

Thursday morning, several Nebraska schools were under lockdown for a second day following an anonymous call that said there would be a shooting at an unspecified school.

A sophomore at a Grand Rapids, Mich., high school was arrested Thursday after a gun police say he brought on campus accidentally went off in a restroom. Also, a high school in Riviera Beach, Md., was evacuated after the school received several threats.

On Wednesday, a high school student in Pikesville, Md., had fired a pellet gun in a courtyard, injuring six boys, police said. And in Jonesboro, Ark., where a 1998 school attack killed five people, a middle school was locked down temporarily after a teacher discovered that a student had a Molotov cocktail in the building.