"We don't monitor across the board every blog posting that may go up," one of the officials said.
Although Defense Department officials take part in terrorism task force efforts, it's unclear how closely the Army worked with the two FBI field offices in tracking Hasan's communications with al-Awlaki or whether Army officials had approached Hasan about the communications.
The people conducting the intelligence investigation never knew that Hasan purchased the gun he allegedly used in the last week's rampage.
"In hindsight, everything looks much clearer ... wish he had decided not to pull the trigger," one senior official said
Because Hasan had no Brady Law exclusions such as mental illness or felony convictions, he was allowed to purchase the gun he allegedly used in the Killeen, Texas, shooting. He bought it in August.
The military's and FBI's prior knowledge of Hasan is expected to get close scrutiny from oversight committees in Congress, with hearings beginning as early as next week in the Senate's Homeland Security and Governmental Affairs Committee.
The FBI and intelligence officials briefed top leadership and staff on the Hasan case Monday evening, according to congressional members.