The press referred to them as military grenades, but then they wouldn’t know a grenade from a racketball. A lot of descriptions are widely overstated, like the press now saying that the 19 yo is a medical student. He may be a smart guy but at 19 I doubt that he has already completed a 4-year undergrad degree and is now a med student.
They may well have obtained real grenades through underground sources, or they may also have had homemade devices. Maybe they received a “grant” of weapons from Fast and Furious.
One of them is now 26, married, 2 kids.