WASHINGTON (AP) – Homeland Security Secretary Jeh Johnson says hackers are taking aim at state and local election officials, and that at least one recent attempt was successful.

Johnson tells MSNBC’s “Morning Joe” on Friday that “exactly who did it” remains under investigation. Last month, the FBI warned state elections officials to boost their election security after hackers targeted data systems in at least two states, Illinois and Arizona. Johnson did not identify a location in his remarks.

Johnson said local election officials should reach out to the federal government to learn how to protect computer systems. With thousands of vote counting systems, “there is no one single point of failure,” he said. But to prevent problems, he said, local officials should be vigilant.