Why were weapons allowed into an interrogation room in Prisoners?

In Prisoners (2013), detective Loki is seen questioning the boy that was drawing mazes while being watched by cops.

At one point, he is tired of waiting for him to give any information about the missing girls, and storms into the interrogation room and assaults the boy, leading to cops intervening and pushing them apart. The boy manages to take a gun from one of the cops and he shoots himself.

I thought that questioning rooms have a basic rule of no-weapons/fire-arms allowed.
Was this a different kind of an interrogation room, or were weapons there for the sake of the plot?