Researchers should determine whether United Nations peacekeepers were the source of a deadly outbreak of cholera in Haiti, two public health experts, including a U.N. official, said Wednesday.

The U.S. Centers for Disease Control and Prevention found that the strain of cholera that has killed at least 442 people the past three weeks matches strains found in South Asia. The CDC, World Health Organization and United Nations say it's not possible to pinpoint the source and investigating further would distract from efforts to fight the disease.

But leading experts on cholera and medicine consulted by The Associated Press challenged that position, saying it is both possible and necessary to track the source to prevent future deaths.

"That sounds like politics to me, not science," Dr. Paul Farmer, a U.N. deputy special envoy to Haiti and a noted expert on poverty and medicine, said of the reluctance to delve further into what caused the outbreak. "Knowing where the point source is — or source, or sources — would seem to be a good enterprise in terms of public health."

The suspicion that a Nepalese U.N. peacekeeping base on a tributary to the infected Artibonite River could have been a source of the infection fueled a protest last week during which hundreds of Haitians denounced the peacekeepers.

John Mekalanos, a cholera expert and chairman of Harvard University's microbiology department, said it is important to know exactly where and how the disease emerged because it is a novel, virulent strain previously unknown in the Western Hemisphere — and public health officials need to know how it spreads.

Interviewed by phone from Cambridge, Massachusetts, Mekalanos said evidence suggests Nepalese soldiers carried the disease when they arrived in early October following outbreaks in their homeland.

"The organism that is causing the disease is very uncharacteristic of (Haiti and the Caribbean), and is quite characteristic of the region from where the soldiers in the base came," said Mekalanos, a colleague of Farmer. "I don't see there is any way to avoid the conclusion that an unfortunate and presumably accidental introduction of the organism occurred."

Cholera, which had never before been documented in Haiti, has killed at least 442 people and hospitalized more than 6,742 with fever, diarrhea and vomiting since late October. It is now present in at least half of Haiti's political regions, called departments.

Death occurs when patients go into shock from extreme dehydration. The epidemic has diverted resources needed for the expected strike of a hurricane this week, and could spread further if there is flooding.

Suspicions that the Nepalese base could have been a source of the infection intensified Monday after the CDC revealed the strain in Haiti matches those found in South Asia, including Nepal.

But nothing has been proven conclusively, and in the meantime the case remains politically charged and diplomatically sensitive. The United Nations has a 12,000-strong force in Haiti that has provided badly needed security in the country since 2004. But their presence is not universally welcomed, and some Haitian politicians have seized upon the cholera accusations, calling for a full-scale investigation and fomenting demonstrations.

Laurie Garrett, senior fellow for global health at the Council on Foreign Relations, said it is clear that the disease was imported to Haiti but that it is still not clear by whom or how. She said the epidemic will contain lessons for humanitarian relief work and disaster relief around the world.

"It has to be either peacekeepers or humanitarian relief workers, that's the bottom line," she said.

Mekalanos said researchers might be more aggressive in finding the source of the infection if the case was less sensitive.

"I think that it is an attempt to maybe do the politically right thing and leave some agencies a way out of this embarrassment. But they should understand that ... there is a bigger picture here," he said. "It's a threat to the whole region."

He also cast doubt on U.N. military tests released this week that showed no sign of cholera. The tests were taken from leaking water and an underground waste container at the base a week after the epidemic was first noted and processed at a lab in the neighboring Dominican Republic, U.N. spokesman Vincenzo Pugliese said.

Mekalanos said that it is extremely difficult to accurately isolate cholera in environmental samples and that false negatives are common. The Nepalese troops were not tested for cholera before their deployment if they did not present symptoms. But health officials say 75 percent of people infected with cholera bacteria do not show symptoms and can still pass on the disease for weeks. A spokesman for the World Health Organization said finding the cause of the outbreak is "not important right now." "Right now, there is no active investigation. I can't say one way or another (if there will be). It is not something we are thinking about at the moment. What we are thinking about is the public health response in Haiti," said spokesman Gregory Hartl. The Harvard experts said more conclusive evidence would be available following closer examinations of the genetic material in the strain. CDC spokeswoman Kathryn Harben said in an e-mail that the center will make the full genomic DNA sequence available when it is confirmed. "At some point in the future, when many different analyses of the strain are complete, it may be possible to identify the origin of the strain causing the outbreak in Haiti," she said. Farmer, who co-founded the medical organization Partners in Health that is a leading responder in the epidemic, said there is no reason to wait. "The idea that we'd never know is not very likely," he said. "There's got to be a way to know the truth without pointing fingers." ___ Associated Press reporter Colleen Barry in Geneva contributed to this article.