Seven U.S. Marines were killed and at least seven other people injured during a training exercise in western Nevada last night, Marine Corps officials announced today.

The Marines, members of a ground combat unit based in Camp Lejeune, N.C., died in an explosion. Marine spokesman Sgt. Jonathan Cress said the training involved live-fire ranges and the handling of ammunition and explosives.

According to witnesses, the accident occurred when a 60-millimeter mortar shell exploded in a tube as Marines were either picking it up to load it or preparing to fire it.

“We send our prayers and condolences to the families of Marines involved in this tragic incident. We remain focused on ensuring that they are supported through this difficult time,” Maj. Gen. Raymond C. Fox, commanding general of the II Marine Expeditionary Force, said in a statement. “We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice.”

The incident took place at an army depot southeast of Reno used for high-desert training for military units and for storing ammunition and weapon stocks awaiting demilitarization. The depot is owned by the government but operated by a contractor.

The cause of the accident is under investigation.

The accident comes on the heels of a new report about Camp Lejeune’s drinking water being contaminated by TCE, an industrial solvent and human carcinogen, as early as 1948 -- five years earlier than researchers previously thought. The time difference is significant, since it affects health care screening and illnesses related to the water. President Barack Obama signed a law last year granting health care and screening to Marines and their dependents on the base between 1957 and 1987.

As many as 1 million people were exposed to tainted water, according to health officials. Some Marines who resided at Camp Lejeune have blamed the contamination for causing cancer in themselves and their family members, including breast cancers in both men and women, bladder cancer and liver cancer.

"Basically, it's vindication and confirmation for what I've been saying for nearly 16 years," said retired Marine Master Sgt. Jerry Ensminger. "The truth is finally coming out."

Ensminger blames the contamination for the leukemia that killed his 9-year-old daughter, Janey, in 1985.

Sen. Richard Burr recently filed to extend coverage back to 1953.