After U.S. intelligence officials intercepted their e-mails, members of two Joint Terrorism Task Forces contacted Major Nidal Malik Hasan's superiors and reviewed his academic and military records for evidence of suspicious activity late last year and early this year, according to three senior U.S. officials. But the investigators concluded that Hasan's activities did not warrant a more formal investigation, even though the imam, Anwar al Awlaki, had ties to al-Qaida operatives and was the author of a popular Web site espousing jihadist activity, the three officials said. The disclosure that Hasan had ongoing communications with Awlaki raised questions as to whether U.S. intelligence and law enforcement agencies had information that, if properly shared and investigated, might have helped to prevent last week's attack. Hasan is accused of firing off more than 100 rounds from a pair of semi-automatic handguns, killing 13 and injuring dozens more. Fifteen people remained hospitalized Monday, with eight in intensive care. In the Hasan 'assessment,' the officials conceded Monday, authorities did not know that he had purchased at least one semiautomatic handgun last summer at a store in Killeen, Texas, even though such purchases go through an FBI check. And there was no indication, the officials said, that investigators knew about an Internet posting from May in which a writer named 'NidalHasan' likened a suicide bomber to a soldier who jumps on a grenade to save the lives of his fellow officers -- in that both were sacrificing their lives 'for a more noble cause.' The officials disclosed those details at a highly unusual briefing Monday evening, after conducting a similar one with Capitol Hill lawmakers. For the most part, they defended investigators, saying they had acted on the best information available at the time. The officials also said Hasan's e-mails to Awlaki appeared mostly innocuous and not worthy of further investigation or monitoring under Justice Department guidelines. But, one of them acknowledged, 'painted in the worst light, in hindsight, someone could reach different conclusions.' FBI Director Robert S. Mueller III has ordered a review of the bureau's 'internal processes.' Maria Gallegos, a spokeswoman for Brooke Army Medical Center in San Antonio, said Hasan has been conscious and talking to medical staff since Saturday. He is in critical but stable condition. Investigators tried to interview him Sunday, but he refused, the officials said. President Barack Obama is scheduled to speak Tuesday afternoon at a memorial service in Fort Hood -- a traditional military affair that includes a sermon, a roll call of the dead and a 21-gun salute. Lt. Gen. Robert W. Cone, the base's commanding general, expected the families of the 13 people killed to be among the 3,000 people in attendance. Even before Monday's disclosures, lawmakers were calling for inquiries into whether the Army, FBI and U.S. intelligence community had missed warning signs about Hasan's increasing radicalization in the months before Thursday's shootings. 'I think the very fact that you've got a major in the U.S. Army contacting this guy (Awlaki), or attempting to contact him, would raise some red flags,' said Rep. Peter Hoekstra of Michigan, the ranking Republican on the House Intelligence Committee. Several officials said U.S. intelligence agencies first intercepted communications between Hasan and Awlaki, a U.S. citizen who left the country in 2002 and is believed to be in Yemen, in late 2008 as a result of another investigation. The information was given to one Joint Terrorism Task Force and then to a second one in Washington because Hasan was living in the area while working and attending classes at Walter Reed Army Medical Center and a military-affiliated health sciences university in Bethesda. 'When reviewed, there was nothing to raise a red flag,' one of the officials said Monday night. The task forces determined that Hasan contacted the radical cleric between 10 and 20 times, but it was mostly 'within the context of the doctor's position and what he was doing at the time, conducting research ... on the issues of Muslims in the military and the effects of war in Muslim countries,' one senior FBI official said earlier Monday. Another official said the research involved post-traumatic stress syndrome and was sanctioned by Hasan's academic overseers. Hoekstra expressed frustration with the handling of the intelligence on Hasan, saying authorities had underestimated the significance of the material they had obtained -- including that Awlaki responded to several of the e-mails. Those responses were regarded by U.S. authorities as 'relatively innocuous,' Hoekstra said. But, he added, 'I think the fact that you're getting responses should have set off red flags regardless of the content.' The emerging details figure to draw parallels with intelligence breakdowns that preceded the Sept. 11 attacks, when the CIA, NSA, FBI and other agencies failed to recognize or share information that may have helped uncover the plot. Fixing those problems was the focus of a sweeping overhaul of the U.S. intelligence community. Congressional investigators 'are going to be taking a look at all of the information and making decisions on whether people should have been notified along the way,' said a congressional official who has been briefed on the Hasan probe. 'I think that's going to depend on the nature of the communications.' Staff writer Ashley Powers in Fort Hood, Texas, contributed to this report. 