Sen. Joe Lieberman's call came as word surfaced that Maj. Nidal Malik Hasan apparently attended the same mosque as two Sept. 11 hijackers in 2001, at a time when a radical imam preached there. Whether Hasan, an Army psychiatrist, associated with the hijackers is something the FBI will probably look into, according to a law enforcement official who spoke on condition of anonymity because the investigation is ongoing.
CBS News correspondent Bob Orr reports that anhas revealed that he did visit Web sites promoting radical Islamic views.
One senior official told CBS News, "Everything continues to indicate that Hassan acted alone." The official, however, described the investigation as, "fluid and still in its early stages."
Sources tell CBS News that investigators believe at this point that Hasan methodically planned his attack and that it appears that he targeted those in military uniform.
Sources say that officials are debating whether to charge him under international terrorism laws in a federal jurisdiction or whether to charge him under military law with murder, CBS News has learned. That is why he has not been charged yet.
Under terrorism an element that would have to be shown is evidence that he was inspired by a terrorist group or influenced or directed by terrorists or terrorist idealogy, CBS News reports. Officials believe at this point that there is mounting evidence that indicates Hasan's actions were inspired by the global jihadist message which would be an element in charging someone with terrorism.
According to those who knew him - including soldiers - Hasan repeatedly proclaimed that the U.S.'s war on terror was a war on Islam and that he wanted nothing to do with his pending deployment to Afghanistan later this month, CBS News correspondent Dean Reynolds reports.
"I don't think anyone suspects at this point, and the authorities aren't suggesting that Hasan was directed by al Qaeda or that he was acting anything other than alone in this attack," CBS News terrorism expert Juan Zarate told "Early Show" co-anchor Harry Smith. "But I think what's troubling is the fact that he did express those views. It appeared he was growing more and more radical, though he was still in his position to treat soldiers. And I think that's the difficult part here for the Army."
Classmates participating in a 2007-2008 master's program at a military college complained repeatedly to superiors about what they considered Hasan's anti-American views. Dr. Val Finnell said Hasan gave a presentation at the Uniformed Services University that justified suicide bombing and even told classmates that Islamic law trumped the U.S. Constitution.
Another classmate said he complained to five officers and two civilian faculty members at the university. He wrote in a command climate survey sent to Pentagon officials that fear in the military of being seen as politically incorrect prevented an "intellectually honest discussion of Islamic ideology" in the ranks. The classmate also requested anonymity because the investigation is ongoing.
More Coverage of the Tragedy at Fort Hood:
Hasan Computer Shows No Terror Ties
List of Fort Hood Dead, Wounded
Army Says Hasan Taken off Ventilator
Fort Hood Suspect Said Methodical Goodbyes
Hasan Reportedly Felt U.S. Attacked Islam
Hasan's Remarks Worried Muslim Leader
Lieberman, an independent who is chairman of the Senate Homeland Security and Governmental Affairs Committee, wants Congress to determine whether the shootings constitute a terrorist attack.
"If Hasan was showing signs, saying to people that he had become an Islamist extremist, the U.S. Army has to have zero tolerance," Lieberman said on "Fox News Sunday." "He should have been gone."
Authorities continue to refer to Hasan, 39, as the only suspect in the shootings that killed 13 and wounded 29, but they won't say when charges would be filed and have said they have not determined a motive. They have not revealed a possible motive. Hasan, who was shot by civilian police to end the rampage, was in critical but stable condition at an Army hospital in San Antonio.
He was breathing on his own after being taken off a ventilator on Saturday, but officials won't say whether Hasan can communicate. Sixteen victims remained hospitalized with gunshot wounds, and seven were in intensive care.
Hasan's family described a man incapable of the attack, calling him a devoted doctor and devout Muslim who showed no signs that he might lash out.
"I've known my brother Nidal to be a peaceful, loving and compassionate person who has shown great interest in the medical field and in helping others," his brother, Eyad Hasan, of Sterling, Virginia, said in a statement Saturday. "He has never committed an act of violence and was always known to be a good, law-abiding citizen."
Army Chief of Staff George Casey warned against reaching conclusions about the suspected shooter's motives until investigators have fully explored the attack. "I think the speculation (on Hasan's Islamic roots) could potentially heighten backlash against some of our Muslim soldiers," he said on ABC television.
Imam Johari Abdul-Malik, outreach director at the Dar al Hijrah Islamic Center, said he did not know whether Hasan ever attended the Falls Church, Virginia, mosque but confirmed that the Hasan family participated in services there. Abdul-Malik said the Hasans were not leaders at the mosque and their attendance was utterly normal.
In 2001, Anwar Aulaqi was an imam, or spiritual leader, at the mosque. Aulaqi told the FBI in 2001 that, before he moved to Virginia in early 2001, he met with 9/11 hijacker Nawaf al-Hazmi several times in San Diego. Al-Hazmi was at the time living with Khalid al-Mihdhar, another hijacker. Al-Hazmi and another hijacker, Hani Hanjour, attended the Dar al Hijrah mosque in early April 2001.
The mosque is one of the largest on the East Coast, and thousands of worshippers attend prayers and services there every week. Abdul-Malik said it's a mistake for people to conflate regular attendance at a mosque with extremism.
Many Muslims pray at the mosque multiple times a day, he said. "It's part of family life. It's like going out for ice cream after dinner."
A government official speaking on condition of anonymity because the person was not authorized to discuss the case said an initial review of Hasan's computer use has found no evidence of links to terror groups or anyone who might have helped plan or push him toward the attack. The review of Hasan's computer is continuing, the official said.
Hasan likely would face military justice rather than federal criminal charges if investigators determine the violence was the work of just one person.
Across the sprawling post and in neighboring Killeen, soldiers, their relatives and members of the community struggled to make sense of the shootings. Candles burned Saturday night outside the apartment complex where Hasan lived. Small white crosses, one for each of the dead, dotted a lawn at a Killeen church on Sunday.
Even as the community took time to mourn the victims at worship services on and off the post, Fort Hood spokesman Col. John Rossi acknowledged that the country's largest military installation was moving forward with its usual business of soldiering.