WASHINGTON (November 6, 2009)--There are many unknowns about Maj. Nidal Malik Hasan, the man authorities say is responsible for the worst mass killing on a U.S. military base in history.

Hasan is the Army psychiatrist authorities say opened fire Thursday afternoon at the post’s Soldier Readiness Center, killing 13 and wounding 30 before a police officer shot him repeatedly.

Hasan reported to Fort Hood in July after working at the Walter Reed Army Medical Center pursuing his career in psychiatry.

He received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001.

At least six months ago, Hasan came to the attention of law enforcement because of Internet postings about suicide bombings and other threats.

Some postings equated suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.

The Associated Press reports officials have not determined for certain whether Hasan is the author of the posting.

Hasan's aunt, Noel Hasan of Falls Church, Va., told The Washington Post that he had been harassed about being a Muslim after the 2001 terror attacks and he wanted out of the Army.

She said he had sought a discharge from the military for several years, and even offered to repay the cost of his medical training.

A military official told AP Hasan was in the preparation stage of deployment, and Hasan had indicated he didn't want to go to Iraq but was willing to serve in Afghanistan.

A neighbor of Hasan's, Patricia Villa, described how on Wednesday morning Hasan offered her some of his items, saying he was going to be deployed.

He provided her some frozen broccoli, some spinach, T-shirts, shelves and a new Quran.

Villa said Hasan returned Thursday morning and gave her his air mattress bed, several briefcases and a desk lamp.

He then offered her $60 to clean his apartment.

Villa said Hasan "seemed like a real nice person."

Col. Kimberly Kesling, deputy commander of clinical services at Darnall Army Medical Center at Fort Hood, said Hasan had provided excellent care to his patients.

Authorities searched his apartment overnight after earlier surrounding the complex in which he lived.

About 20 other residents were evacuated.

They were taken to the Killeen Community Center.