First of two articles on Army's criminal investigation into brutal deaths of two detainees at detention center in Bagram, Afghanistan; 2,000-page confidential file depicts young, poorly trained soldiers in repeated incidents of abuse; finds that in some instances, it was directed or carried out by interrogators to extract information, that sometimes it was punishment meted out by military police guards, and that torment sometimes seemed driven by little more than boredom or cruelty, or both; finds one detainee, who had been chained to top of his cell by his wrists for many days, was taken for last abusive interrogation when most of interrogators believed he was innocent; so far, only seven soldiers have been charged; most of those who could still face legal action have denied wrongdoing; story of abuses at Bagram remains incomplete, but documents and interviews reveal striking disparity between findings of Army investigators and what military officials said in aftermath of two deaths; detailed description of treatment of two detainees; photos; excerpts from statements by various officers.