Russian government hackers have penetrated the computer network of the Democratic National Committee and gained access to all opposition research on Republican presidential candidate Donald Trump, the committee and security researchers say.
Representative Debbie Wasserman Schultz, chairwoman of the Democratic National Committee, on Tuesday confirmed the breach to Reuters, first reported by the Washington Post.
"When we discovered the intrusion, we treated this like the serious incident it is," Wasserman Schultz said in a statement.
"Our team moved as quickly as possible to kick out the intruders and secure our network."
The Washington Post quoted US officials as saying Russian spies also targeted the networks of Trump and Democratic presidential candidate Hillary Clinton, as well as computers of some Republican political action committees.
The intrusion is emblematic of the sophistication of Russian hackers, who intelligence officials have long viewed as the most talented of US adversaries in cyberspace.
In Moscow, a Kremlin spokesman did not immediately respond to a request for comment.
Two separate groups were able to enter the DNC's system and read email and chat communications, according to the committee and CrowdStrike, the cyber firm that helped clean up the breach.
CrowdStrike began assisting the DNC in May and identified two groups, said Dmitri Alperovitch, the company's co-founder and chief technology officer.
Both were kicked out this weekend, he said.
The first, which CrowdStrike named Cozy Bear, entered the DNC's systems last year, according to the firm.
It primarily monitored email and chat conversations and might have been working for Russia's Federal Security Service, or FSB, Alperovitch said.
FSB was once run by Russian President Vladimir Putin.
The second group, nicknamed Fancy Bear, is probably working on behalf of Russia's military, Alperovitch said.
It gained entry in late April and "went straight to the oppo research specifically on Donald Trump and exfiltrated some of it", he said.