Fort Hood, Texas (CNN) -- Investigators believe the suspected gunman in last week's massacre at Fort Hood acted alone, but his communications had been flagged by U.S. intelligence agencies in late 2008, the FBI said Monday.



The suspect, Maj. Nidal Malik Hasan, remained in intensive care at Brooke Army Medical Center in San Antonio, Texas. In a statement issued Monday night, the FBI said its investigation so far "indicates that the alleged gunman acted alone and was not part of a broader terrorist plot."



Thursday's shooting left 13 dead, 12 of them U.S. soldiers, and 42 wounded.



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Hasan, a U.S.-born citizen of Palestinian descent, was a licensed psychiatrist who joined the Army in 1997. He was promoted to major in May and was scheduled to deploy to Afghanistan sometime soon, but had been telling his family since 2001 that he wanted to get out of the military.



A Muslim, he had told his family he had been taunted after the terrorist attacks of September 11, 2001. In August, he reported to police that his car was keyed and a bumper sticker that read "Allah is Love" was torn off. A neighbor was charged with criminal mischief after that complaint.



But the FBI disclosed that Hasan came to its attention as part of an unrelated terrorism probe in December 2008, when agents reviewed "certain communications between Maj. Hasan and the subject of that investigation."



The intercepts "raised no red flags," with no mention of threats or violence that would have triggered a U.S. terrorism investigation, senior investigative officials said Monday.



Hasan, 39, was wounded several times during the attack. Though still in intensive care, his ventilator was removed over the weekend, and he began talking afterwards, hospital spokesman Dewey Mitchell said.



Federal agents attempted to interview Hasan on Sunday, but he refused to cooperate and asked for an attorney, the investigative officials said.



U.S. military officials said intelligence agencies intercepted communications between Hasan and Anwar al-Awlaki, a former imam at the Dar Al-Hijrah Islamic Center in Falls Church, Virginia, a Washington suburb. Al-Awlaki, who left the United States in 2002 and is believed to be living in Yemen, was the subject of several federal investigations dating back to the late 1990s, but was never charged.



Military officials told CNN on Monday that intelligence agencies intercepted communications from Hasan to al-Awlaki and shared them with other U.S. government agencies. But federal authorities dropped the inquiry into Hasan's communications after deciding that the messages warranted no further action, one of the officials said.



According to the FBI, investigators from one of its Joint Terrorism Task Forces determined "that the content of those communications was consistent with research being conducted by Maj. Hasan in his position as a psychiatrist at the Walter Reed Medical Center [in Washington]."



Hasan was first an intern, then a resident and finally a fellow at Walter Reed before moving to Fort Hood.



"Because the content of the communications was explainable by his research and nothing else derogatory was found, the JTTF concluded that Maj. Hasan was not involved in terrorist activities or terrorist planning," it said.



FBI Director Robert Mueller has ordered a review of the matter, the FBI said.



The independent commission that investigated the 9/11 attacks found al-Awlaki was a "spiritual adviser" to two of the hijackers in that plot, Nawaf al-Hazmi and Khalid al-Midhar, while al-Awlaki was at the Virginia mosque and earlier, in San Diego.



The commission report said it was not clear whether the imam knew al-Hazmi and al-Midhar were involved in the hijacking plot, but security experts have described him as a radical Islamic fundamentalist who was "very supportive of terrorists in the past," former White House homeland security adviser Frances Townsend told CNN.



An online post attributed to al-Awlaki praised Hasan as a hero for the Fort Hood attack, saying he "could not bear living the contradiction of being a Muslim and serving in an army that is fighting against his own people.
What is the condition of the attacker What did the new FBI probe on Fort Hood reveal Who is awake at the hospital? What has the probe shown so far?
wounded several times during the attack. Though still in intensive care, his ventilator was removed over the weekend, and he began talking afterwards, the alleged gunman acted alone and was not part of a broader terrorist plot." Maj. Nidal Malik Hasan, FBI disclosed that Hasan came to its attention as part of an unrelated terrorism
question: What is the condition of the attacker, answer: wounded several times during the attack. Though still in intensive care, his ventilator was removed over the weekend, and he began talking afterwards, | question: What did the new FBI probe on Fort Hood reveal, answer: the alleged gunman acted alone and was not part of a broader terrorist plot." | question: Who is awake at the hospital?, answer: Maj. Nidal Malik Hasan, | question: What has the probe shown so far?, answer: FBI disclosed that Hasan came to its attention as part of an unrelated terrorism