The Army psychiatrist, a devout American-born Muslim, is believed to have acted alone, despite repeated communications – intercepted by authorities – with a radical imam overseas, US officials said.
The FBI will conduct an internal review to see whether it mishandled early information about the man accused in the bloody rampage that killed 13 people and wounded 29.
President Barack Obama joined grieving families and comrades of the victims yesterday at a memorial service at the sprawling Texas Army base.
Hasan, awake and talking to doctors, met his lawyer on Monday in the San Antonio hospital where he is recovering, under guard, from gunshot wounds.
In Washington, an official and a Republican lawmaker said Hasan had communicated 10 to 20 times with US-born Anwar al-Awlaki, an imam released from a Yemeni jail last year who has used his personal website to encourage Muslims across the world to kill US troops in Iraq.
Despite that, no formal investigation was opened into Hasan, they said.
Al-Awlaki is a former imam at a Falls Church, Virginia, mosque where Hasan and his family occasionally worshipped.
Pete Hoekstra of Michigan, the top Republican on the House Intelligence Committee, said it was his understanding Hasan and the imam exchanged emails that counter-terrorism officials picked up.
Officials said Hasan will be tried in a military court, not a civilian one, a choice that suggests his alleged actions are not thought to have emanated from a terrorist organisation.
Meanwhile, The Washington Post reported yesterday that Hasan warned his medical colleagues a year and a half ago that to “decrease adverse events” the US military should allow Muslim soldiers to be released as conscientious objectors instead of fighting in wars against other Muslims.
Hasan, an Army psychiatrist, made the recommendation in a culminating presentation to senior Army doctors at Walter Reed Medical Center, where he spent six years as an intern, resident and fellow before being transferred to Fort Hood.
“It’s getting harder and harder for Muslims in the service to morally justify being in a military that seems constantly engaged against fellow Muslims,” Hasan said in the presentation.
FBI director Robert Mueller ordered the inquiry into the bureau’s handling of the case. Based on all the investigations since the attack, the investigators said they have no evidence Hasan had help or outside orders in the shootings.
Military officials were made aware of communications between Hasan and al-Awlaki, but because the messages did not advocate or threaten violence, civilian law enforcement authorities could not take the matter further, the officials said. The terrorism task force concluded Hasan was not involved in terrorist planning.
Officials said the content of those messages was “consistent with the subject matter of his research”, part of which involved post-traumatic stress disorder stemming from US combat operations in Iraq and Afghanistan.
A law enforcement official said the communications consisted primarily of Hasan posing questions to the imam as a spiritual leader or adviser, and the imam did respond to at least some of those messages.
Officials said the decision to bring military charges instead of civilian criminal charges against Hasan did not mean it wasn’t a terrorism case. But it is likely authorities would have had more reason to take the case to federal court if they had found evidence Hasan acted with the support or training of a terrorist group.
Investigators tried to interview Hasan on Sunday but he refused to answer and requested a lawyer, the officials said.