Take a look at the police:
In Gaza, apparently anyone with a gun (and a ski mask) can now legally do police work. There is no indication that Hamas is against this police force and in fact Hamas works closely with Islamic Jihad in other matters. It seems likely that Hamas allows the Islamic Jihad terrorists to act openly as an official security force.
When "human rights groups" claim that the police in Gaza are considered civilians, keep this in mind.
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