The chief campaign organization of the Republican Party has been hacked and thousands of emails from senior aides compromised over the period of several months, it has emerged.

In an incident reminiscent of the notorious cyber-attack on the Democratic National Committee (DNC) ahead of the 2016 presidential election, three GOP officials said the National Republican Congressional Committee had been breached.

According to Politico, the incident was discovered in April but not related even to senior lawmakers like speaker Paul Ryan and majority leader Kevin McCarthy until the news site contacted them this week.

“We don’t want to get into details about what was taken because it’s an ongoing investigation,” a senior party official is quoted as saying. “Let’s say they had access to four active accounts. I think you can draw from that.”

The committee is said to believe it could be the work of a foreign agent, although those rumors haven’t been confirmed. The FBI is currently investigating.

President Trump has in the past gloated at the fact that Republicans did not suffer a data breach while the DNC was compromised, claiming it is because the GOP has better cybersecurity rather than the Russian hackers were out specifically to smear his opponent in the race for the White House.

Brian Vecci, technical evangelist at Varonis, said the attackers appear to have targeted individuals’ inboxes specifically to gain intelligence.

“The countries that have proven themselves to be able to perpetuate these kinds of attacks and have the motive to do so are Russia, China, and North Korea,” he added. “That doesn’t rule anyone else out, it just means they’re the most likely. These are the countries with the means, motive, and opportunity to get the most out of inside information related to US political thinking.”