Supposedly, there is an inquiry which found absolutely nothing could have been done to prevent this.
Typically, body armour worn by police (and guards) is level IIA, it will stop a shotgun or .357 Magnum pistol........but rifle bullets.......no way. To stop rifle rounds requires stiff, heavy solid ceramic plates in the armour.......which make it very difficult to wear day after day.
These guys were just there watching the shed, stolen car parts and a couple of grass plants......big deal!
Two had been there all night, one was just arriving to relieve them, one was off-duty (and unarmed) but was just dropping his friend off.....
The rifle was a semi-auto H&K 91 (.308 Winchester)
An absolute tragedy.