WASHINGTON, Sept. 7 - Tests of water covering New Orleans showed excessive levels of E. coli bacteria and lead, federal officials said Wednesday, providing the first confirmation that the floodwaters caused by Hurricane Katrina are posing health risks for emergency response workers and residents who have remained in the city.

While neither substance has been blamed for any deaths, a spokesman for the Centers for Disease Control and Prevention said state and local officials had reported three deaths in Mississippi and one in Texas from exposure to Vibrio vulnificus, a choleralike bacteria found in saltwater that poses special risks for people with chronic liver problems.

The centers' spokesman, Tom Skinner, said a fifth case involving the bacteria was under investigation.

The water tests, conducted by the Environmental Protection Agency at laboratories in Houston and Lafayette, La., sought to identify the more than 100 chemical and bacterial pollutants in two water samples from six sites in largely residential areas over four days, starting Saturday.