The two-year review, led by former FBI Director William Webster, concluded FBI agents' mistakes were unintentional and they should not be held responsible or punished for failing to prevent the November 2009 shootings, the San Antonio Express-News reported.
The review found FBI agents on the San Diego Joint Terrorism Task Force were aware Hasan had contacted known terrorist Anwar al-Awlaki in Yemen numerous times before the Fort Hood shootings and discussed the killing of civilians. But the review said the FBI agents did not bring the e-mails to the attention of the Defense Department.
In one e-mail, Hasan, an American-born Muslim, discussed suicide bombers and whether it would be permissible for "the killing of innocents for a valuable target."
Al-Awlaki, an American Muslim from New Mexico, was killed in 2011 in a U.S.-launched drone strike. He had moved to Yemen and become an al-Qaida operative.
The review said an FBI field office in Washington determined Hasan, an Army psychiatrist who had served at Walter Reed Army Medical Center, was not involved in terrorist activities.
Agents did not interview Hasan, co-workers or supervisors who had become aware of his increasing radicalization.
He wasn't interviewed, an agent told the FBI in San Diego, because the Washington field office "doesn't go out and interview every Muslim guy who visits extremist Web sites." The agent said the subject was "politically sensitive."
The review makes 18 recommendations, including new training procedures, a review of protocols of operations and information systems, and corrective measures to improve investigative techniques and reporting.
Hasan is accused of entering the Soldier Readiness Center at Fort Hood on Nov. 5, 2009, and opening fire with two pistols. He is in jail awaiting court-martial on 13 charges of premeditated murder and 32 charges of attempted premeditated murder.