From 2001 to 2004, Washington, D.C. experienced what may have been the worst lead contamination of city water on record. Tens of thousands of homes had leaded water coming out of their taps -- in the worst cases, the tap water contained enough lead to be classified as hazardous waste.

But a 2004 CDC report claimed to find that the water contamination "might have contributed a small increase in blood lead levels." The study has been influential. Officials in New York and Seattle have used the CDC report as justification for not aggressively responding to high levels of lead in their water, and other cities have cited the report to dispel concerns about lead in tap water.

But the results of thousands of blood tests that measured lead contamination in children were missing from the report. What’s more, the CDC discovered in 2007 that many young children living in D.C. homes with lead pipes were poisoned by drinking water and suffered ill effects, such as speech and balance problems, difficulty with learning, and hyperactivity. Yet the health agency did not publicize the new findings or alert public health authorities.

CDC scientists and press representatives did not respond to requests for an explanation about why the results were not widely publicized.