Explosion during a training exercise or was it something else?

I always take anything and everything coming from the US media with a pinch of salt.  I have sprinkled a lot of it on this story because I fail to understand why a training exercise would be conducted late in the evening.

From FoxNews Charlotte:
...An explosion at a military ammunition storage facility in Nevada during a Marine Corps training exercise killed seven U.S. Marines and wounded several others, military officials said.

The North Carolina-based Marines with the 2nd Marine Division were killed late Monday when a 60-millimeter mortar exploded during a live-fire training exercise at Hawthorne Army Depot as they were preparing to fire it, NBC News reports.

The cause of the incident remains under investigation. The identities of those killed were not released pending notification of their families, officials said in a statement from the 2nd Marine Expeditionary Force at Camp LeJeune, N.C.

"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," said Maj. Gen. Raymond C. Fox, the force's commander. "We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."

Stacy Kendall, a spokeswoman for Renown Regional Medical Center, told NBC News the facility was treating eight people wounded in the blast. Three were listed in serious condition; five were listed in fair condition. Kendall said the injuries included traumas and fractures.....