Army Maj. Nidal Malik Hasan, 39, is accused of opening fire at a soldier readiness center in Texas, killing 13 people and wounding dozens more.
A senior federal law enforcement official told the Los Angeles Times Monday that there is no immediate connection between Hasan and terrorist groups, but that all options are being explored.
A high-ranking official told the Times on the condition of anonymity that authorities are searching "voluminous" hard drives, e-mail accounts and Web traffic history "to see what's out there, and to see what it all means. There's a lot of work being done."
Already authorities have reportedly found evidence that Hasan frequently visited radical Islamic Web sites. Officials are also exploring a possible connection to the former imam of a mosque in Northern Virginia, who is known for his fiery sermons and blog posts.
Early Monday morning, Imam Anwar al Awlaki, who is now living in Yemen, reportedly posted on a Web site that Hasan was "a man of conscience who could not bear living the contradiction of being a Muslim and serving in an army that is fighting against his own people," according to the Associated Press.
Two U.S. intelligence officials told the AP that the Web site was Awlaki's. They spoke on condition of anonymity to discuss intelligence collection. Awlaki did not immediately respond to an attempt to contact him through the Web site, according to the AP.
But law enforcement officials said on Sunday that, as of now, the suspect is believed to have worked alone.
"There is nothing that we have found thus far linking him to a terrorist group, but we have intelligence that he made postings in chat rooms," said Scott Stewart, vice president of tactical intelligence for Stratfor, a private firm that gathers intelligence for divisions of the U.S. and foreign governments. "Everything that I'm seeing so far is that he was a lone wolf."
Sen. Joe Lieberman, I-Conn., who heads the Senate Homeland Security Committee, said that he wants to launch a congressional investigation into the attack.
"If Hasan was showing signs, saying to people that he had become an Islamic extremist, the U.S. Army has to have a zero tolerance," he told "Fox News Sunday." "He should have been gone."
Two unnamed U.S. officials told ABC News on Monday that intelligence agencies were alerted months ago that Hasan had attempted to contact al-Qaida, according to the ABC News Web site. It remains unclear whether the agencies informed the Army at the time or whether the reports have been confirmed.
Media reports are also shedding new light on Hasan's background and the hours before the deadly attack. According to the New York Times, those close to Hasan indicated that he increasingly had become disillusioned with his role in the military.
The morning of the shooting, Hasan attended early morning prayers at a mosque near Fort Hood, according to the Times.
"I'm going traveling," he reportedly told another worshipper. "I won't be here tomorrow."
On Sunday, Army Chief of Staff George W. Casey Jr. urged troops not to act out against other Muslims serving in the military.
"I'm concerned that this increased speculation could cause a backlash against some of our Muslim soldiers," Casey told CNN's "State of the Union." And I've asked our Army leaders to be on the lookout for that."
Hasan remains hospitalized. He began talking on Sunday, and is said to be communicating with hospital staff.
Sixteen victims also remain in the hospital with seven in intensive care
President Barack Obama is scheduled to attend a Tuesday memorial service at Fort Hood.
Meanwhile, Sgt. Kimberly Munley, the police officer who exchanged gunfire with the attacker last week, is still hospitalized, recovering from wounds to her thighs and her right wrist, the New York Times reported.
Before helping to bring the Fort Hood attack to an end, she was credited for saving her police partner's life when she was working in Wrightsville Beach, N.C.
Investigator Shaun Appler, Munley's former partner, spoke with the Online NewsHour about she saved his life and how she earned the nickname "Mighty Mouse":
---- Media reports with interview by Dave Gustafson