WASHINGTON • United States 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 US 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 US spy agencies had concluded that two Russian intelligence agencies or their proxies were targeting the Democratic National Committee (DNC), the central organising 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 Federal Bureau of Investigation said it was investigating a cyber attack at the DNC. The DNC did not respond to a request for comment.
The congressional briefing was given last summer in a secure room to a group of congressional leaders nicknamed the "Gang of Eight", the sources said.
This group includes four Republicans: Senate Majority leader Mitch McConnell and House of Representatives Speaker John Boehner, and Senator Richard Burr and Representative Devin Nunes, the House and Senate intelligence committee chairs.
Their Democratic counterparts are Senator Harry Reid and Representative Nancy Pelosi, and Senator Dianne Feinstein and Representative Adam Schiff of the intelligence committees.
DNC officials have said they did not learn of the hacking until months after the initial congressional briefing, when an agent from an FBI cyber-security squad asked them last autumn about the party's data security arrangements. Even then, the Democratic sources said, the FBI agent never mentioned that US intelligence officials suspected that Russian hackers were targeting the organisation.
The attack on the DNC later led the hackers to other party organisations, including the Democratic Congressional Campaign Committee, which raises funds for House candidates, and Mrs Hillary Clinton's presidential campaign. The hackers ini- tially used "spearphishing" - attacks on e-mail accounts of dozens of people at the organisations, sources said.