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 on 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 codeword, 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.
The National Security Agency and other intelligence agencies sometimes delay informing targets of foreign intelligence activities under similar circumstances, officials have said.
The alleged hacking of the Democrats and the Russian connection did not become public until late last month when the FBI said it was investigating a cyber attack at the DNC. The DNC did not respond to a request for comment for this story.
The congressional briefing was given last summer in a secure room called a Sensitive Compartmented Information Facility, or SCIF, to a group of congressional leaders informally known as the "Gang of Eight," the sources said.
This group includes four Republicans: Senate Majority leader Mitch McConnell and House of Representatives Speaker Paul Ryan, and Senator Richard Burr and Representative Devin Nunes, the House and Senate intelligence committee chairs.