WASHINGTON: His name appears on radical Internet postings. A fellow officer says he fought his deployment to Iraq and argued with soldiers who supported US wars. He required counseling as a medical student because of problems with patients.

There are many unknowns about Nidal Malik Hasan, the man authorities say is responsible for the worst mass killing on a US military base. Most of all, his motive. But details of his life and mindset, emerging from official sources and personal acquaintances, are troubling.

For six years before reporting for duty at Fort Hood, Texas, in July, the 39-year-old Army major worked at the Walter Reed Army Medical Center pursuing his career in psychiatry, as an intern, a resident and, last year, a fellow in disaster and preventive psychiatry. He received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Maryland, in 2001.