Sen. Joe Lieberman announced Sunday that he intends to lead a congressional investigation into the mass shooting at Fort Hood, saying the attack could qualify as a "terrorist act" rooted in Islamic radicalism -- the worst since 9/11. 

The Independent Democrat, who chairs the Senate Committee on Homeland Security and Governmental Affairs, said there were "strong warning signs" that the alleged gunman, Maj. Nidal Malik Hasan, was an "Islamist extremist." 

"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," Lieberman told "Fox News Sunday." 

The Connecticut senator said authorities "don't know enough" yet, but said his panel would investigate the gunman's motives and "ask whether the Army missed warning signs that should have led them to essentially discharge him." 

Lieberman said that if Hasan were showing warning signs, "The U.S. Army has to have zero tolerance. He should have been gone." 

In interviews Sunday, Army Chief of Staff Gen. George Casey urged the public not to rush to conclusions about Hasan's motives with an investigation underway. He described reports about early warning signs as "speculation" based on anecdotes. 

"I don't want to say that we missed it," he said on NBC's "Meet the Press." 

The attacks on Fort Hood in Texas last Thursday left 13 people dead and 29 wounded. 

The rampage sparked a debate about whether it could be viewed as an act of terrorism. 

The alleged gunman, a Palestinian-American and an Army psychiatrist, reportedly shouted "Allahu akbar! -- Arabic for "God is great!" when he opened fire. He was seriously wounded by police and is being treated in a military hospital. 

The military has said he was scheduled to deploy to Afghanistan, and family members suggested he was trying to avoid serving overseas. 

That doesn't add up to the prelude to a terrorist attack, said Carl Tobias, a professor of law at University of Richmond who analyzes terrorist investigations across the country. 

"Terrorist attacks are undertaken by people who typically ... have some agenda they want to forward politically, and from what I see in the news, this is just a person acting individually because he doesn't want to deploy overseas," he said. "So I just don't see that angle." 

But others disagreed. 

Lieberman said bystanders' accounts of the attack raise "genuine concerns that this was a terrorist act."