Poor procedure. The guards/cops should never enter a cell with a firearm.
Looks like in this case they didn’t bother to search the perp before putting him in the cell. The surviving cops need to go to The Barney Fife Police Academy.
Second sentence in excerpt: "Louis De Zoysa, 25, pulled out an antique revolver"
It's even worse procedure to not thoroughly search somebody you are locking up.