There can be no enemy worse than the enemy within.

Last week’s killing spree by an Army psychologist at Fort Hood, Texas, increasingly looks like the work of an enemy within the “Army family” that should have provided safety and security at the huge military installation. And it should spur the military to look harder at its own, to watch even more carefully for signs of danger.

In this case, the emerging portrait of Maj. Nidal Malik Hasan includes not just the accused shooter’s growing distress with an upcoming deployment to Iraq, but his growing expression of anti-American views and reported links to a Virginia mosque at a time when inflammatory talks were given by a radical imam who had met with at least one 9/11 hijacker before the attacks on the World Trade Center.

“If Hasan was showing signs, saying to people that he had become an Islamist extremist, the U. S. Army has to have zero tolerance,” Connecticut Sen. Joseph I. Lieberman rightly noted recently. “He should have been gone.”

There are about 3,500 Muslims serving in America’s military, all but one or two of them serving honorably. Hasan, the only suspect in the 13 Fort Hood killings, is an exception. So was Sgt. Hasan Akbar, who used grenades and rifle fire to kill two officers and wound 14 other members of his own unit in the opening days of the Iraq invasion—some five years after he wrote in his diary, “My life will not be complete unless America is destroyed.”

The military has been recruiting Muslims actively since 9/11, offering incentives largely in an effort to gain translators. There now is a mosque on the Naval base in Norfolk, and an all-Muslim translation unit based in California. Because military personnel are not required to list religious preferences, some think the number of Muslims now serving may be 10,000 higher than the official count.

Any backlash against Muslims honorably serving this country would be unfair. But ignoring the fact that part of Islam has been hijacked by Islamist extremists, and that those extremists could reach into the ranks of the military, would be unwise.

Islamic groups should lead the condemnation of such Islamist misuse of the religion, loudly. But they should not view legitimate vigilance as illegitimate discrimination. The military should be extremely careful with its treatment of its own—but it has a duty, one regrettably necessitated by Hasan and Akbar, to protect its own troops, Muslims as well as non-Muslims, against potential risks. If the Army missed warning signs this time, it must make sure it does not do so again.

The Fort Hood shootings not only shattered lives, they shattered the confidence of troops in harm’s way around the world that at least their families were protected and secure in the safety of a home-soil military base. The investigations of motive must be exhaustive, and the risks properly and thoroughly evaluated. There can be no witch hunts—but there must be deep and thorough screening, and development of better but fair reporting and warning systems, to make as certain as humanly possible that no single group within the military either launches such attacks or suffers them.