WASHINGTON — The FBI and the Pentagon are responsible for a "string of failures" in the way they attempted to track a disgruntled Army major in the years before he allegedly opened fire at a crowded Ft. Hood, Texas, deployment center in the worst domestic terror ambush since the attacks of September 2001, two key Senate leaders concluded Thursday.

In addition, Army supervisors repeatedly referred to Maj. Nidal Hasan as a "ticking time bomb," and FBI agents and the military knew he had become radicalized under the influence of a violent Islamist extremist. Yet the agents never arrested him, and his military superiors never disciplined or furloughed him out of the Army.

"The Ft. Hood massacre should have been prevented," said Sen. Joe Lieberman (I-Conn.), who along with Sen. Susan Collins (R-Maine) conducted the investigation into the November 2009 shooting on behalf of the Senate Homeland Security Committee.

"People in the Department of Defense and the FBI had ample evidence of alleged killer Nidal Hasan's growing sympathies toward violent Islamist extremism in the years before the attack. He was not just a ticking time bomb but a traitor. Thirteen people died needlessly at Ft. Hood."

Hasan, a 41-year-old U.S.-born Muslim of Palestinian descent who worked as an Army psychiatrist, reportedly yelled, "Allahu Akbar" — Arabic for "God is great" — when he burst into the Soldier Readiness Center and allegedly opened fire. Besides the 13 deaths, 32 people were wounded.

Hasan was shot outside the center and was paralyzed from the chest down. The Army is considering sending him to a general court-martial, where he could face the death penalty.

The Senate committee leaders launched their investigation to determine what went wrong in the Hasan case and how future "lone-wolf terrorists" could be spotted and dealt with.

Along the way, the FBI, the Department of Justice and the Pentagon all tussled with the senators, sometimes refusing to provide critical documents that shed light on Hasan's past and what they knew about him. At one point, the committee threatened subpoenas.

Lieberman and Collins said their "basic conclusion" was that the FBI and the Defense Department never had specific information of a time or place when Hasan might attack. But, they said, the agencies "collectively had sufficient information to have detected Hasan's radicalization to violent Islamist extremism but failed to understand and to act on it."

Furthermore, they said, "our investigation found specific systemic failures in the government's handling of the Hasan case and raises additional concerns about what may be broader systemic issues."

The bottom line, they said, was that "the FBI and DoD together failed to recognize and to link the information that they possessed about Hasan."

They determined that federal law-enforcement agents, "to the FBI's credit," did flag Hasan for additional scrutiny by the FBI after learning of his radicalization.

Much of that occurred after Hasan had contacts, often by e-mail, with Anwar Awlaki, an American-born Yemen-based Islamic cleric with suspected ties to Al Qaeda.

"I can't wait to join you" in the afterlife, Hasan once reportedly e-mailed Awlaki. After the shootings, Awlaki praised Hasan as a "hero" and a "man of conscience … serving in an Army that is fighting against its own people."

Hasan's radicalization "was on full display to his superiors and colleagues during his military medical training," the Lieberman-Collins report said. "An instructor and a colleague referred to Hasan as a 'ticking time bomb.' "

Yet "not only was no action taken to discipline or discharge him, but also his Officer Evaluation report sanitized his obsession with violent Islamist extremism into praiseworthy research on counterterrorism."

An FBI Joint Terrorism Task Force learned of his radicalization and passed it on to another FBI task force. "However, the ensuing inquiry failed to identify the totality of Hasan's communications and to inform Hasan's military chain of command and Army security officials of the fact that he was communicating with a suspected violent Islamist extremist."

The Hasan e-mails to Awlaki, known to preach violence and accused of encouraging others to kill for Al Qaeda, should have immediately been seen as "a shocking course of conduct for a U.S. militaryofficer." Instead, the FBI read his evaluation report and agreed that he was probably only doing research.

The FBI also decided the evidence against Hasan was "slim," and agents "dropped the matter rather than cause a bureaucratic confrontation," the report said.

Even officials at FBI headquarters in Washington "never acted" on the evidence. "As a result, the FBI's inquiry into Hasan ended prematurely."

Had the evidence been investigated further, and shared with counter-intelligence officials, the report concluded, "this critical mistake may have been avoided."

richard.serrano@latimes.com