The Army psychiatrist accused of the Fort Hood massacre apparently acted alone and without outside direction in the attack, investigative officials said Monday evening.

Maj. Nidal Malik Hasan will be charged by the U.S. military rather than in a civilian court, they said.

Hasan communicated 10 to 20 times with a radical imam overseas who in the past came under scrutiny for possible links to terror groups, the investigative officials and a U.S. official disclosed. The investigative officials said the communications began last year and continued into this year and "were consistent with the subject matter of his research. The U.S. official said the communications were with the imam, Anwar al-Awlaki.

U.S. officials were aware of the communications since last year, but no formal investigation was ever opened based on them, the officials said.

The officials spoke Monday evening on condition of anonymity because they were not authorized to discuss the investigation on the record. Awlaki, who was released from a jail in Yemen last year, writes a blog that denounces U.S. policies as anti-Muslim.

Investigators tried to interview Hasan on Sunday at the military hospital where he is held under guard, but he refused to answer and requested a lawyer, the officials said.