WASHINGTON—Hillary Clinton used more than a dozen email devices during her time as secretary of state, and a technician took steps to delete an archive of her emails after House lawmakers demanded they be saved, according to documents released Friday by the Federal Bureau of Investigation.

The newly public information also shows that Mrs. Clinton was warned at the outset of her tenure by former Secretary of State Colin Powell that her work-related email messages could become subject to public release. And in an interview with FBI agents in July, the Democratic presidential candidate offered a defense of her handling of sensitive drone-strike conversations.

The new disclosures were contained in two documents released by the FBI on Friday—a report summarizing the bureau’s investigation of Mrs. Clinton’s email arrangement, which concluded with a recommendation that she not be prosecuted, and a summary of her interview. The FBI said it was releasing the material in the interests of transparency.

On Friday, Clinton campaign spokesman Brian Fallon said, “We are pleased that the FBI has released the materials from Hillary Clinton’s interview, as we had requested.” He added, “While her use of a single email account was clearly a mistake and she has taken responsibility for it, these materials make clear why the Justice Department believed there was no basis to move forward with this case.”

The documents disclosed few dramatic new facts, but they painted a picture of Mrs. Clinton as inattentive to computer security and unsophisticated about the classification system. They also confirmed a Wall Street Journal report in June that the FBI was especially concerned about email exchanges including Mrs. Clinton that concerned possible drone strikes.