Veterans of the Gulf War suffer more health problems than other members of the military but the causes of the mysterious array of symptoms may never be known, a leading British scientist said on Monday.

Men and women who served in the 1990-1991 war are 20 percent more likely to suffer from headaches, fatigue and pain but do not have a higher rate of cancer or heart disease.

“There is no shadow of a doubt that something has happened, something has gone wrong,” Professor Simon Wessely, of King’s College London, told a news briefing.

But the head of the Gulf War research unit at the college said the increase in ill health is unlikely to be a new disease or have a single cause.

“There are huge areas that remain unclear and I am afraid I suspect they will always remain unclear,” he added.

Multiple vaccinations, exposure to pesticides, smoke from oil-burning fires, stress and organophosphates, chemicals that have been shown to affect the human nervous system, have been cited as possible causes of the illness.

Professor Mark Peakman, an immunologist at Guy’s, King’s & St Thomas’ School of Medicine in London, said the human immune system is more than capable of dealing with multiple vaccinations given in a short period of time.

Although vaccinations can cause local reactions, there is no evidence so far linking them with the symptoms of Gulf War Syndrome.

Professor David Ray of Nottingham University doubted that exposure to dangerous chemicals in pesticides or insecticides was the culprit.

“It doesn’t look like a pattern of chemical poisoning,” Ray said.

Wessely and his colleagues spoke to journalists ahead of the release of an independent British inquiry into the illness and a day after a draft U.S. report by a group of doctors and veterans said exposure to neurotoxins explains the symptoms better than stress or psychiatric illness.

The British scientists said they could not comment on the U.S. report without seeing it.