Why wasn't the SAS used to rescue Linda Norgrove ( British Aid worker in killed in Afghanistan)?

Maybe so but I'm wondering why after three others had been released that they went in at all.The leader of a 22-strong Afghan delegation, dispatched by the Afghan authorities to talk to the insurgents,said he was confident that they could have won her freedom. As Cameron O.K'd it he carries the can.Could be that the Americans already had special forces in the area.Granted it does seem deferential.