U.S. intelligence officials told top congressional leaders a year ago that Russian hackers were attacking the Democratic Party, three sources familiar with the matter said Thursday, but the lawmakers were unable to tell the targets about the hacking because the information was so secret.

The disclosure of the top-secret information would have revealed that U.S. intelligence agencies were continuing to monitor the hacking, as well as the sensitive intelligence sources and the methods they were using to do it.

The material was marked with additional restrictions and assigned a unique code word, limiting access to a small number of officials who needed to know that U.S. spy agencies had concluded that two Russian intelligence agencies or their proxies were targeting the Democratic National Committee, the central organizing body of the Democratic Party.