The State Department is investigating episodes in which one or more people used artificial intelligence to impersonate Secretary of State Marco Rubio in messages to top foreign diplomats and U.S. officials, according to an official at the agency.

Mr. Rubio’s office sent a cable, or an agency memo, last week to State Department employees about the efforts to impersonate him.

When asked on Tuesday about the cable and the episodes, the department’s press office issued a statement from a senior official that said the department was “aware of this incident and is currently investigating the matter.”

“The department takes seriously its responsibility to safeguard its information, and continuously takes steps to improve the department’s cybersecurity posture to prevent future incidents,” the statement said.

The person or people impersonating Mr. Rubio sent text and voice messages, including over the encrypted chat app Signal, to at least five people outside the State Department. The recipients included a governor, a member of Congress and three foreign ministers, according to the cable, which was reported earlier by The Washington Post. The cable was dated July 3.

Mr. Rubio’s voice and mannerisms can be observed on many online sites. He is a ubiquitous presence on television, holds news conferences and appears in many photo opportunities in which he exchanges words with other diplomats. Before becoming secretary of state this year, he was a senator from Florida who spoke frequently at congressional hearings and other public appearances, including when he ran against Donald J. Trump for the Republican presidential nomination in 2016.