Washington — U.S. Sens. Richard Burr and Kay Hagan and Congressman Brad Miller on Friday sent a letter to Navy Secretary Ray Mabus demanding more openness when dealing with the public about contaminated drinking water at Camp Lejeune.

The lawmakers questioned the Navy's and Marine Corps' "commitment to transparency and veracity in efforts to keep the public informed" about efforts to study the problem and deal with health problems many veterans and their families attribute to the water.

Government officials have said an estimated 750,000 people may have been exposed to cancer-causing substances in the Marine base’s water supply between the 1950s and 1980s.

The lawmakers wrote that they are concerned with the Marine Corps’ referrals to a 2009 report by the National Research Council, which downplays the link between the contaminated water and adverse health affects. The NRC report didn't assess the presence of benzene, a known carcinogen, in the water, but Marines Corps officials continue to tell the public that benzene was evaluated, the lawmakers said.

The Agency for Toxic Substances and Disease Registry is conducting scientific studies to understand the extent of exposure on Camp Lejeune and is planning human health surveys to determine the frequency of certain illnesses among former base residents.

Burr and Hagan have co-sponsored legislation to provide care for veterans and their family members who have experienced health problems because of the contaminated Camp Lejeune water.