Washington (CNN) -- A key congressional committee opened its investigation Thursday into the November 5 Fort Hood shootings with a pledge to find out if authorities failed to "connect the dots" and could have prevented the attack.



The head of the committee promised the inquiry would not interfere with a separate investigation into the shootings being conducted by Defense Secretary Robert Gates and Attorney General Eric Holder.



"Their investigation looks backward and is punitive. Ours looks forward and is preventive," said Senate Homeland Security Committee Chairman Joe Lieberman, I-Connecticut,



Maj. Nidal Malik Hasan, a U.S. Army psychiatrist, is accused of opening fire at a military processing center at Fort Hood Army Post, killing 13 people. Dozens of others were wounded.



Hasan was shot and paralyzed during the attack and remains in a military hospital.



Lieberman said the committee will carry out its investigation "with respect for the thousands of Muslim-Americans who are serving in the American military with honor and the millions of other patriotic, law-abiding Muslims who live in our country."



But, he added, "we do no favor to all our fellow Americans who are Muslim by ignoring real evidence that a small number of their community have, in fact, become violent Islamists and extremists."



Lieberman said the committee's investigation will focus on whether concerns raised by Hasan's colleagues about his "mental stability and political extremism" were dealt with appropriately by senior Army officials.



Among other things, a memo reportedly written two years ago by Hasan's supervisor at Walter Reed Army Medical Center says Hasan demonstrated "a pattern of poor judgment and a lack of professionalism" during his residency at the hospital.



CNN could not corroborate the authenticity of the memo, which was obtained by National Public Radio.



Lieberman also said the committee will examine a lack of response by federal authorities after uncovering an exchange of e-mails between Hasan and a radical cleric accused of having ties to al Qaeda.



The FBI has said it was aware of communication between Hasan and Anwar al-Awlaki, a Yemeni-American cleric who has promoted jihad against the United States and other Western countries. But investigators determined that those contacts were "consistent with research being conducted by Maj. Hasan."



Also, Lieberman noted, the committee will look into whether information on Hasan gathered by a joint terrorism task force was shared with officials in the Army, the Defense Department or elsewhere.



Maine Sen. Susan Collins, the ranking Republican on the committee, said Hasan's case "raises questions about whether or not restrictive rules have a chilling effect on the legitimate dissemination of information, making it too difficult to connect the dots that would have allowed a clear picture of the threat to emerge."



But a law enforcement official familiar with the investigation told CNN that when Hasan first came to the attention of investigators because of his communications with al-Awlaki, officials looked at his military personnel file and nothing was found that raised suspicion.



None of the items that have been reported since the shootings -- including the reported memo from his Walter Reed supervisor or a Power Point presentation arguing that Muslims in the Army should be given conscientious objector status -- were part of the file, the official said, but it was noted that Hasan had done research about Muslims in the military.



Thursday's hearing included testimony from former Army Vice Chief of Staff Gen. John Keane, Rand Corp. consultant Brian Jenkins and New York City Police Department intelligence analyst Mitchell Silber.



"At a glance, Maj. Hasan's rampage at Fort Hood looks a lot like what used to be called 'going postal,' " Jenkins said. It was "a deepening sense of personal grievance culminating in a homicidal rampage directed against co-workers -- in this case, fellow soldiers.



"For Hasan, 'going jihad' reflects the channeling of obvious personality problems into a deadly fanaticism," he said.



The committee launched its investigation only hours before Gates announced a 45-day review of Pentagon policies to see if the Defense Department has fallen short in identifying service members "who could potentially pose credible threats to others
what happened at fort hood What has been investigated? When did the massacre happen? who did they need to share the information with? What did Sen. Joe Lieberman say about the incident? Who is opening an inquiry into the massacre?
shootings 5 Fort Hood shootings 5 officials in the Army, "Their investigation looks backward and is punitive. Ours looks forward and is preventive," A key congressional committee
question: what happened at fort hood, answer: shootings | question: What has been investigated?, answer: 5 Fort Hood shootings | question: When did the massacre happen?, answer: 5 | question: who did they need to share the information with?, answer: officials in the Army, | question: What did Sen. Joe Lieberman say about the incident?, answer: "Their investigation looks backward and is punitive. Ours looks forward and is preventive," | question: Who is opening an inquiry into the massacre?, answer: A key congressional committee