WASHINGTON -- There may be additional e-mails that could have tipped off law enforcement or military officials to the Fort Hood shooter before he went on his deadly rampage, the chairman of the Senate Armed Forces Committee said Friday.

Democratic Sen. Carl Levin said after a briefing from Pentagon and Army officials that his committee will investigate how those and other e-mails involving the alleged shooter, Maj. Nidal Malik Hasan, were handled and why the U.S. military was not made aware of them before the Nov. 5 shooting.

The U.S. government intercepted at least 18 e-mails between Hasan and Anwar al-Awlaki, a radical American-born cleric. They were passed along to two Joint Terrorism Task Force cells led by the FBI, but a senior defense official said no one at the Defense Department knew about the messages until after the shootings. The official spoke on condition of anonymity to discuss intelligence procedures.

Levin said his committee is focused on determining whether the Defense Department's representative on the terrorism task force acted appropriately and effectively.

Levin also said he considers Hasan's shooting spree, which killed 13 and wounded more than 30, an act of terrorism.

"There are some who are reluctant to call it terrorism, but there is significant evidence that is. I'm not at all uneasy saying it sure looks like that," he said.

He said his committee also will look into whether military members have the ability to report suspicious behavior evinced by colleagues.

FBI and military officials have provided differing versions of why Hasan's critical e-mails to al-Awlaki and others did not reach Army investigators before the shooting.

FBI officials have said a military investigator on the task force saw the e-mails and looked up Hasan's record. Finding nothing particularly worrisome, the investigator neither sought nor got permission to pass the e-mails on to other military officials.

The senior defense official has countered that the rules of the task force prevented that military representative from passing the records on without approval from other members of the task force.

Sen. Joe Lieberman, chairman of the Senate Homeland Security Committee, said it appears there was enough information available to law enforcement, the military and intelligence agencies to raise alarm bells about Hasan but no one connected the dots.

"Had it been gathered on one desk, someone might have said `Nidal Malik Hasan is dangerous,"' Lieberman, an independent, told reporters after the briefing.

The Pentagon may reconsider rules governing participation in extremist organizations that some lawmakers say appear outdated and too narrow in light of the shooting rampage at the Army base in Texas.

Lieberman said Congress may recommend such a review, and a Pentagon spokesman said Friday that the rules could be among the policies scrutinized by a wide-ranging inquiry aimed at preventing another similar attack.

The Pentagon wrote regulations on "dissident and protest activities" in response to soldier participation in skinhead and other racially motivated hate groups. The current rules were written in 1996 and last updated in 2003.

The rules prohibit membership or participation in "organizations that espouse supremacist causes," seek to discriminate based on race, religion or other factors or advocate force or violence. Commanders can investigate and can discipline or fire people who "actively participate in such groups."

The rules also cover the distribution and possession of "printed materials," and gatherings held outside military posts.

The language appears loosely to cover some of the activity law enforcement sources have ascribed to Hasan.

It is geared, however, toward racially motivated groups and toward preventing public espousal of hateful ideology, such as attendance at a rally or the recruitment of new members. The language also applies most directly to materials and communication in the pre-Internet age.

Defense Secretary Robert Gates announced the 45-day probe on Thursday, the same day that retired Army Gen. John Keane told Congress that the existing rules will probably need revision to cover activity of "Islamic extremists."

Any revision would have to be done carefully to avoid First Amendment violations on the free exercise of speech and religion.

Keane was formerly the No. 2 Army official.

The Pentagon inquiry will get under way in earnest next week.

A senior military official said the inquiry's top leaders will meet with Gates on Monday and are likely to visit Fort Hood on Tuesday. The official spoke on condition of anonymity because plans are not final.