51 Terror Suspects Detained

Just yesterday, 51 suspects were detained during security operations in Iraq. This was unfortunately accompanied by the death of a US soldier.

"The U.S. soldier died of gunshot wounds after his patrol came under small-arms attack in northeast Baghdad. The incident is under investigation, and his name is being withheld pending notification of next of kin. "

Stupid question but why does this need investigating, surely the reason was pretty simple..

Link to article

However I am unable to find out how many in total are currently detained by allied forces (before you ask, I have googled and can only find details on individual cases.) but with numbers like the above shown, I would imagine the number to be incredibly high.. how many detainee sites and how many guards would be required to house them all.. I am aware how much money os spend on average prison facilities for civilians, surely the cost of detaining the terror suspects would be extensive? surely it is not possible to arrest a whole nation of people..