WASHINGTON (Nov. 15) - The Beltway sniper, the University of Arizona gunman, the Fort Bragg murders, the Oklahoma City bomber.

The terrible and unfathomable crimes behind the headlines vary widely but all share a common thread that researchers say may merit a closer look: With the exception of one of the four Fort Bragg killings, all are alleged to be have been committed by veterans of the 1991 Gulf War.

There are too many unanswered questions to draw broad conclusions about whether the men connected with these crimes were suffering from the illnesses that research has shown afflict some 25 to 30 percent of the 697,000 U.S. Gulf veterans.

However, studies have turned up evidence of injury to the brain in some ill veterans of the conflict, including damage to the deep brain structures where personality is determined.

What caused this damage, and other symptoms veterans describe, isn't clear, but researchers have said possibilities could include environmental toxins, low-level nerve agents, depleted uranium, oil fires, mustard gas, stress as well as vaccines given to soldiers to guard against biological warfare and nerve gas.

Dr. William Baumzweiger, a California neurologist and psychiatrist who specializes in Gulf War ailments, said he was not surprised that so many of the high-profile crimes were tied to Gulf veterans. ''Gulf War veterans have a very high frequency of turning to violence to deal with frustration,'' he said.

A TERRIBLE TOLL

Baumzweiger testified for the defense at the trial of Gulf veteran Jeffrey Hutchinson, convicted last year of the 1998 murders of his girlfriend and her three children in Florida.

But Hutchinson does not win the prize for infamy in this group. That goes to Timothy McVeigh, executed in 2001 for the 1995 bombing of a federal building in Oklahoma City that killed 168 people and injured hundreds of others.

September and October of this year brought two more high-profile cases involving veterans.

John Allen Muhammad, along with a young accomplice, has been accused of killing 10 people in and around Washington D.C. He is also charged with shootings in Louisiana and Alabama and could be linked to others.

Then in late October, failing Arizona nursing student Robert Flores, who served in the Army during the Gulf War, mowed down three of his professors before shooting himself.

Earlier in 2002, four servicemen allegedly killed their wives at Fort Bragg in North Carolina. Three of the four were Gulf War veterans.

Last week, a military team probing the Fort Bragg deaths blamed marital woes, deployment stress and reluctance to seek counseling.

''REASONABLE HYPOTHESIS''

Privacy Act rules make it impossible to find out if any of the Gulf veterans in these high-profile crimes ever officially complained of symptoms, and researchers are unaware of any statistics that indicate that rates of violence among Gulf veterans are higher than the general populace or than other combat veterans.

One researcher, who declined to be identified, said of speculation about a link between Gulf War illnesses and the crimes: ''It's a very reasonable hypothesis and it's reasonable because these people came back with personality change, difficulty controlling anger and so forth.''

''The question is over 10 years, what is the expected incidence of violent shooters, violent criminals, in the population of 695,000 former military people? I don't know the answer to that. Nobody knows...although these are such high-profile crimes, you'd expect that the incidence of that would be extremely rare,'' he added.

Steve Robinson, executive director of the National Gulf War Resource Center, a veterans' advocacy group, said more study of Gulf War ailments is clearly needed.

''Do Gulf War veterans as a whole demonstrate psychotic, homicidal, suicidal behavior? I don't think so. Are there individuals that have demonstrated those? Yes, absolutely,'' he said, adding that while the vast majority of those who suffer from Gulf War ailments will never turn violent, he receives despairing letters and telephone calls daily from sufferers.

In an emotion-choked voice, Robinson read from one such letter, written by a veteran in jail for a vehicular homicide that killed a close friend. It said in part: ''I'm nervous all the time. I feel like my body is doing 200 miles an hour. I am always fatigued, my body shakes and sweats. I believe that because of the physical symptoms, I am a basket case. Anxiety and depression rule my life.''

NOT JUMPING TO CONCLUSIONS

According to the Bureau of Justice Statistics, in 1999 -- the latest year for which the data are available -- just 16 people aged from 25 to 49 committed murder per 100,000 population.

There is no breakdown according to military service.

''There is no evidence to support the notion that Gulf War veterans are more violent than any other group,'' said Barbara Goodno, a spokeswoman at the Defense Department.

''We should be careful not to jump to conclusions. Approximately 697,000 veterans served their country in operations Desert Shield and Desert Storm. It would be an injustice to them to automatically link the aberrant acts of a few to their military service,'' she added.

But enough questions linger that with the country teetering on the brink of another conflict with Iraq, researchers think these violent crimes may merit further study.

''These high-profile shooters, that looks like it could be something new. And certainly the Gulf War personality change thing could account for it,'' the researcher said.

The U.S. government does not acknowledge a Gulf War ''syndrome'' -- a group of signs and symptoms adding up to a unique condition. It admits there are a number of illnesses that have emerged in veterans of the conflict but until recently it has put these down to psychology.

Symptoms can include difficulty with concentration, thinking and memory, severe body pain, chronic diarrhea, sleep disturbances, night sweats, hot flashes and personality change, said Dr. Robert Haley of the University of Texas Southwestern Medical Center, a member of the research advisory council on Gulf War illnesses to the Department of Veterans Affairs.

''It's common for these guys to have become (different),'' Haley said. ''Their wives will tell you, 'This isn't the guy who went over. He's had a personality change.' And they typically come back (with) difficulty controlling temper, often depressed, withdrawn, not wanting to be around other people, difficulty dealing with complex environments.''

Haley said it is ''too big a leap'' to go from this to a conclusion that Gulf War brain injuries could be prompting this small group of men to commit terrible crimes.

POTENTIAL BREAKTHROUGH

According to a report the advisory committee issued to the Department of Veterans Affairs in June, the ailments of veterans of the relatively short conflict ''cannot be adequately explained by deployment stress, wartime trauma or psychiatric diagnoses such as post-traumatic stress disorder.''

The report said neurological problems are a key category of Gulf War illnesses and that there is enough evidence ''to conclude that this line of inquiry represents a potential breakthrough that could be pursued.''

Last month, the department issued a statement citing the research on a possible neurological link and committing $20 million in fiscal 2004 to further study. The department will set up a brain-imaging center to probe the issue.

''It's not inconceivable that certain individuals may have severe neurological impairment,'' said veterans' advocate Robinson. ''I can't sit here and tell you that that's the reason they commit crimes.