An underwater bombs was found at the base of the Converse Reservoir dam in Mobile, Alabama, a dam that holds the drinking water for the entire city. The explosive device was discovered by Mobile Area Water and Sewer System divers on Wednesday during a routine inspection for maintenance and repairs. They said it was a “grenade-type IED.”

Federal law enforcement from the FBI Bomb Squad joined the Mobile County Sheriff’s Office, Mobile Police Department Explosive Ordnance Detail, ALEA Bomb Squad, and Daphne Search and Rescue Team in investigating. The bomb was detonated off site.

DHS was also notifies as the Converse Reservoir dam, built in 1952, and Big Creek Lake, holding 17 billion gallons of water, are critical infrastructure per a federal government designation. Bomb experts said they believed the IED was intentionally put at the base of the dam.

“Our top priority is keeping your drinking water safe. This is an unprecedented threat, and we are fortunate that this device was discovered before it could cause serious damage to our water supply or harm to individuals,” said Mobile Area Water and Sewer System director Bud McCrory.

“We are grateful for the professionalism and competency of our law enforcement partners – as well as the quick thinking of our contractors and divers – in identifying this device and safely destroying it,” he continued, per the Daily Mail. He said they would “work with relevant law enforcement agencies to enhance security at the reservoir and dam.”

Surveillance cameras around the dam monitor activity but a suspect has not yet been named. 9,000 acres of land surround the dam and lake to protect the water supply.