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The following is a rush transcript of the November 8, 2009, edition of "FOX News Sunday With Chris Wallace." This copy may not be in its final form and may be updated.

CHRIS WALLACE, HOST: Now to our other top story, that terrible shooting spree at Fort Hood Thursday. There are still more questions than answers about the worst attack ever at a U.S. military base.

Did the gunman, Army Major Nidal Hasan, work alone? What was his motive? And should the tragedy have been prevented? For answers we turn to Joe Lieberman, chairman of the Senate Homeland Security Committee. And, Senator Lieberman, welcome back, sir.

SEN. JOE LIEBERMAN, I-CONN.: Chris, good to be with you.

WALLACE: In the briefings that you and your staff have received, have you learned any more about Major Hasan's motives, his actions, and whether or not that he had any links to Islamic radicals overseas?

LIEBERMAN: It's — first, this was a terrible tragedy. Second, it's too early — it's premature to reach conclusions about what motivated Hasan. But it's clear that he was, one, under personal stress and, two, if the reports that we're receiving of various statements he made, acts he took, are valid, he had turned to Islamist extremism.

And therefore, if that is true, the murder of these 13 people was a terrorist act and, in fact, it was the most destructive terrorist act to be committed on American soil since 9/11.

But I want to say very quickly we don't know enough to say now, but there are very, very strong warning signs here that Dr. Hasan had become an Islamist extremist and, therefore, that this was a terrorist act.

WALLACE: I'm going to pursue that in a second. But any evidence so far that what you or your staff have heard in briefings that he — because we know he was on some radical Islamic Web sites...

LIEBERMAN: Right, right.

WALLACE: ... that he was exchanging communications either in this country or overseas with other Islamic radicals?

LIEBERMAN: Yeah. Nothing I can confirm at this point. I think it's very important to let the Army and the FBI go forward with this investigation before we reach any conclusions.

But what we do know on the record from third parties reporting over the last two or three years — that he made a series of statements justifying suicide bombing, comparing it to the bravery of an American soldier who would throw himself on a grenade to protect his colleagues, that he said that — well, he shouted out, according to bystanders at that — while killing the other day at Fort Hood, the words Allah Akbar, an expression of faith in Islam which the Islamist extremists have corrupted.

And the fact that he did that at the moment of these murders — if that's confirmed, of course — raises genuine concerns that this was a terrorist act.

I will add to this, Chris, this is not the first attempt by Islamist extremists to strike at American military bases. We've broken up plots to go after Fort Dix, Quantico Marine base in Virginia.

In fact, the one successful, if I can put it that way, terrorist act that was done in recent years was the individual in Little Rock, Arkansas who walked into an Army recruiting station and killed a recruiter.

And there is testimony that Dr. Hasan actually said that he understood that and supported that act.

WALLACE: A lot of people are wondering — you talk about all the statements he made. There were a lot of warning signs out there. I know hindsight is 20/20, but were there enough signs that — enough red flags that authorities should have stepped in?

LIEBERMAN: Well, that's a very important question. And I would say, Chris, that while the Army and the FBI are conducting the criminal investigation about exactly what happened and what Dr. Hasan should be charged with, the U.S. Army — the Department of Defense has a real obligation to convene an independent investigation to go back and look at whether warning signs were missed, both of his — the stress he was under, but also the statements that he was making which really could lead people to believe that Dr. Hasan had become an Islamist extremist.

A couple of years ago, after a two-year investigation, my committee put out a report that said the new face of terrorism in America would not just be the attacks as 9/11, organized abroad and sending people in here. It would be people within this country, home- grown terrorists, self- radicalized, often over the Internet, going to jihadist Web sites.

And there's concern from what we know now about Hasan that, in fact, that's exactly what he was, a self-radicalized home-grown terrorist.

WALLACE: I've got a couple of questions I want to get in, so...

LIEBERMAN: Please. Go ahead.

WALLACE: As chairman of the Senate Homeland Security Committee, do you intend — with all these questions out there, do you intend to hold hearings?

LIEBERMAN: I intend, working with my Republican ranking member Susan Collins, to begin an investigation. I think the first steps that should be taken in this regard should be taken by the U.S. Army, because this was an attack on American troops. You've got to see it as if 12 American troops were killed in Afghanistan.

WALLACE: But you're intending to hold your own congressional...

LIEBERMAN: I am intending to begin a congressional investigation of my Homeland Security Committee into what were the motives, what were the motives of Hasan in carrying out this brutal mass murder, if a terrorist attack, the worst terrorist attack since 9/11, and to ask whether the Army missed warning signs that should have led them to essentially discharge him.

Really, in the U.S. Army, this is not a matter of constitutional freedom of speech. If Hasan was showing signs, saying to people that he had become an Islamist extremist, the U.S. Army has to have zero tolerance. He should have been gone.

WALLACE: Finally — we have less than a minute left — the House passed health care. What do you think of the bill they passed? And do you still intend, if there is a public option and if there's this tax on so- called Cadillac health plans...

LIEBERMAN: Right.

WALLACE: ... will you support a Republican filibuster on final passage in the Senate?

LIEBERMAN: Well, there's some good things in the House-passed plan. You know, I'm — I think we ought to do health care reform this year to deal with the two great problems that President Obama and others have talked about.

There are unsustainable continuing increases in the cost of health care. We've got to — we've got to stop that. And there are millions of Americans who don't have health insurance.

But I'm afraid our colleagues in the House added a lot onto that that subtract from the genuine purposes of health care reform, and one was to create a public option plan.

The public option plan is unnecessary. It has been put forward, I'm convinced, by people who really want the government to take over all of health insurance. They've got a right to do that. I think that would be wrong.

But worse than that, we have a problem even greater than the health insurance problems, and that is a debt — $12 trillion today, projected to be $21 trillion in 10 years.

WALLACE: So at this point, I take it, you're a "no" vote in the Senate.

LIEBERMAN: If the public option plan is in there, as a matter of conscience, I will not allow this bill to come to a final vote, because I believe the debt can break America and send us into a recession that's worse than the one we're fighting our way out of today. I don't want to do that to our — to our children and grandchildren.

WALLACE: Senator Lieberman, thank you. Thanks for coming in today and joining us.

LIEBERMAN: Thank you, Chris.

WALLACE: And we'll be following your congressional investigation, sir.

LIEBERMAN: Good.

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