When the Centers for Disease Control and Prevention published new guidelines 18 months ago regarding the radiation risk from cellphones, it used unusually bold language on the topic for the American health agency: We recommend caution in cellphone use." The agency's website previously had said that any risks "likely are comparable to other lifestyle choices we make every day." Within weeks, though, the C.D.C. reversed course. It no longer recommended caution, and deleted a passage specifically addressing potential risks for children. More than 500 pages of internal records obtained by The New York Times, along with interviews with former agency officials, reveal a debate and some disagreement among scientists and health agencies about what guidance to give as the use of mobile devices skyrockets. Although the initial C.D.C. changes, which were released in June 2014, had been three years in the making, officials quickly realized they had taken a step they were not prepared for. The new guidelines ... aroused alarm within the agency, and concerns from some outside experts. An official from the Vermont Health Department forwarded a letter he had received asking about the state's legal liability for allowing wireless technology in public schools and libraries. Within the C.D.C., officials began to retreat from the language. In emails, Robert C. Whitcomb Jr., head of the Radiation Studies Branch, began assuring colleagues at other agencies and universities that the new guidelines were "not an official policy."