SAN JUAN, Puerto Rico — A U.S. agency has overturned its 2003 research that said no health hazards were caused by decades of military exercises on Vieques, a bombing range-turned-tourist destination off Puerto Rico's east coast.

The federal Agency for Toxic Substances and Disease Registry said Friday it intends to "modify" some of its earlier research on Vieques, where the U.S. and its allies trained for conflicts from Vietnam to Iraq.

The agency, a part of the Centers for Disease Control and Prevention, used its own studies to conclude in 2003 that there was essentially no health risk from the bombing range — a conclusion widely criticized by academics and residents on the 18-mile-long island of less than 10,000 people.

"We have identified gaps in environmental data that could be important in determining health effects," director Howard Frumkin said in a statement posted Friday on the agency's Web site. "The gaps we found indicate that we cannot state categorically that no health hazards exist in Vieques. We have found reason to pose further questions."

Frumkin, who did not specifically identify the research gaps, said the agency will take a fresh look at the issue. He also said the agency will work with Puerto Rican health officials to conduct more in-depth health evaluations and will recommend monitoring to determine if Vieques residents were exposed to harmful chemicals.

For decades, warships and planes hammered the Naval Training Range on Vieques with live rounds before it was closed in April 2003 after years of protests over environmental risks and the 1999 death of a Puerto Rican civilian guard killed by an errant bomb.

Robert Rabin, who moved to Vieques from Boston in 1980 and helped lead the protests against the bombing range, said he and other islanders had an "attitude of cautious celebration" about the agency's announcement.

"We hope this will lead to the best possible cleanup and allow people here to receive the best health care," Rabin said Saturday from his Vieques home. "They are using hopeful language, and this island really needs help."

The U.S. agency reevaluated its earlier finding after being asked in April by U.S. Rep. Steve Rothman, a New Jersey Democrat who said independent studies and reports had documented a health crisis on Vieques.

The military fired and dropped millions of pounds of bombs, rockets and artillery shells, including napalm, depleted uranium and Agent Orange, on Vieques. A cleanup began in 2005 to clear thousands of unexploded munitions from the former range, which is now a Fish and Wildlife Service refuge, and the island has placed new emphasis on tourism.

Some 7,000 past and current Vieques residents have filed a federal lawsuit seeking billions of dollars in compensation for illnesses they have linked to the bombing range.