As the Pentagon investigates the airstrike that killed 22 civilians on the Doctors Without Borders hospital in Kunduz, Afghanistan, last weekend, it’s crucial that authorities be as transparent as possible – not only about what the pilots and spotters on the ground did, but also about the rules they’re required to follow.

The attack came from an AC-130 gunship, which has traditionally not been subject to the “collateral damage estimation” procedure that air controllers use to determine whether fighter jets and drones get the OK to attack.

The reason for the exemption, at least according to a copy of the collateral-damage guidelines obtained by the ACLU in 2009, is that the gunship has highly accurate “direct fire” weapons and is considered far less likely to cause civilian deaths than a high-speed jet dropping bombs.

Were the pilots and ground troops in Kunduz operating under these looser rules of engagement? More broadly, are there good arguments for allowing the AC-130 and other direct-fire craft more freedom to engage? The answers to both of these questions will be hard to get – because the military keeps its rules of engagement classified.

A public explanation of what happened in Kunduz will be a challenge for the Pentagon, but it’s the minimum requirement after a mistake of this magnitude. Even more important is an assessment of the military’s rules for protecting civilians. And for that debate, people outside the Pentagon need to know exactly what those rules are and how they can be improved.