It's a police/SWAT training range on the grounds of the nuclear plant.
At least five bullets escaped the firing range and traveled more than a half-mile before striking buildings and a vehicle near the reactors, according to the NRC, Constellation and the sheriff's office.
One struck the plant's "outage control center," which is used as a command area to orchestrate refueling efforts. Another hit an employee's sport-utility vehicle in the parking lot. Three others struck an office facility: Two of them hit the roof, and one shattered the outer pane of a first-floor window.
Employees were working in both buildings at the time, said Maureen Brown, a Constellation spokeswoman. The bullets did not penetrate either structure, she said.
"No one was injured. . . but it's our range and our responsibility, and we take this very seriously," Brown said. "We are investigating very thoroughly, and we're confident that no bullets left the plant property."
NRC spokeswoman Holly Harrington said the agency was briefed on the incident but was not conducting its own investigation. "This wasn't a [plant] safety or security issue, so we're really not involved," she said.
Firing ranges are common on the sprawling grounds of the nation's nuclear facilities, Harrington said. At Calvert Cliffs, the range is used about 200 days a year by plant security officers, who are tested regularly by commission auditors.
Brown said the range is used by county sheriff's deputies and is the site of a major annual SWAT exercise. "It's part of our community outreach to coordinate with local law enforcement," she said.
On May 14, SWAT officers from various police agencies were practicing on the plant's firing range, which is west of the reactors.
Officers from Calvert, Charles and Prince George's counties, Baltimore and the U.S. Marshals Service were working in teams of three practicing "room clearing" exercises, in which they try to quickly assess dangerous situations in small places and shoot while they are moving, said Lt. Steve Jones, commander of criminal investigations for the sheriff's office.
All handled quietly and professionally. But you know the hysteria that would arise if this were a private range?