The U.S. Army Corps of Engineers on Friday began mailing letters to nearly 500 Suffolk County property owners who live near former military defense sites, warning that unexploded munitions, such as bombs and rockets, could be on or near their property.

The letters are going to homeowners who live within the boundaries of a former Air Force base and bombing/gunnery range near what now is Gabreski Airport in Westhampton Beach, Fort Michie on Great Gull Island in the Town of Southold and Fort H.G. Wright on Fishers Island.

“Available information indicates military munitions may be present on or near your property as a result of past munitions-related activities . . .” the letter said. “If present, these military munitions may pose an explosive hazard to you, your family, other property users or the public.”

The letter instructs people to be aware of their surroundings and to follow the “3 Rs of explosive safety,” which include recognizing munitions, retreating from the item and reporting the discovery to law enforcement.

“Local law enforcement will arrange for Department of Defense Explosive Ordnance Disposal or police bomb squad personnel to remove and destroy it,” the letter said.

The notices stem from the Formerly Used Defense Sites program, created by Congress in the mid-1980s, which charged the Army and the Army Corps of Engineers with identifying and investigating legacy contamination at sites that had been sold and redeveloped. Nationwide an estimated 10,000 sites could be in the program. Cleanup projects are planned or under way at 2,700.

Long Island has 17 former defense sites from Sands Point to Montauk. At least eight may have possible explosives on site.

Inspections at the Suffolk sites identified in the letters sent Friday had indicated explosives may be present, but plans to investigate won’t occur for at least five years because of higher-priority sites and the need for funding, said Heather Sullivan, manager of the Formerly Used Defense Sites program for the corps’ New England Region, which oversees New York.

The region, which includes New Jersey, has a backlog of about $400 million in cleanup and assessment needs and received $21 million for fiscal year 2017, she said.

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The letters were not prompted by the discovery of explosives. The agency is required to notify residents every five years about a site’s history if no work is planned.

“There may be some people who don’t know,” Sullivan said. “We don’t think there is any reason to panic or we would be out there working.”

Most of the letters were mailed to 456 properties in Westhampton at the former Suffolk County Army Air Field Range Complex, a roughly 12,000-acre area that was used for bombing, strafing and rocket-fire training decades ago. Fighter pilots also used it as a gunnery training range.

The sites are listed on the state Department of Environmental Conservation’s Superfund list, but the Army Corps of Engineers is responsible for cleanup. Sullivan said the corps consulted with the DEC and Suffolk County about where to send letters.

On Friday, in a separate matter, the DEC announced that work had started to connect homes with private wells near Gabreski Air National Guard Base to the municipal water system after the chemical perfluorooctanesulfonic acid, or PFOS, was detected earlier this year.

The Air National Guard base was added to the state Superfund list in September, and the DEC will monitor the munitions issue as it conducts investigations related to the PFOS contamination, DEC spokesman Sean Mahar said.

The zones where munitions may be located and where the water-system work is being done do not overlap, said Peter Scully, Suffolk County deputy county executive for administration.

“We continue to monitor the situation,” he said. “As far as we know, it’s more north of the area that we’re focusing on for water hookups.”