Only one of two grenades that were found at a Santa Monica apartment building storage unit last week detonated the following day in East Los Angeles -- killing three sheriff's detectives -- but the second grenade is missing,

The deadly blast was caused by 1 of 2 grenades retrieved the day before, and believed to be inert, Sheriff Robert Luna confirmed. The 2nd one has not turned up, despite extensive searches.

A grenade has not been found and remains missing after the explosion at an East L.A. training facility that killed three deputies, the sheriff says.