WASHINGTON — Sophisticated hackers linked to Russian intelligence services broke into the Democratic National Committee’s computer networks and gained access to confidential emails, chats and opposition research on presumptive Republican nominee Donald Trump, people familiar with the breach said Tuesday.

The firm, CrowdStrike Inc., said the DNC asked it to investigate a suspected breach of its systems, which began as early as last summer. CrowdStrike said it quickly found traces of two of the best adversaries in the hacking arena, both tied to the Russian government.

The newly revealed attacks join a host of high-profile digital breaches affecting current and past White House hopefuls, underscoring vulnerabilities in digital networks that increasingly hold sensitive data about political candidates, their opponents and their donors.

DNC Chairwoman Debbie Wasserman Schultz called the incident serious and said the committee moved quickly to “kick out the intruders and secure our network.” The DNC said donor, financial and personal information did not appear to have been accessed by the hackers.

But an individual knowledgeable of the breach said at least one year’s worth of detailed chats, emails and opposition research on Trump were stolen. That kind of research, a staple of political campaigns, often contains detailed information — sometimes factual and sometimes specious — about a candidate’s personal and professional history.

The individual, who spoke on condition of anonymity because the person was not authorized to discuss the breach, said DNC officials first learned about the hack in late April when its technology staff discovered malware on its computers.

CrowdStrike reported Tuesday that one group of hackers was able to execute computer code remotely on systems running Microsoft Windows. Another was capable of recording keystrokes.

Director of National Intelligence James Clapper said last month that U.S. officials have seen indications of foreign hackers spying on the presidential candidates. He said the U.S. intelligence community expects more cyber threats against the campaigns.

Foreign hacking was rampant during the 2008 presidential election, and President Barack Obama and Mitt Romney were targets of Chinese cyberattacks in the 2012 campaign. In 2008, Obama and Republican nominee Sen. John McCain were also targeted.

CrowdStrike said one of the hacking groups identified in the DNC attack, dubbed Cozy Bear, had previously infiltrated unclassified networks at the White House, the State Department and the Joint Chiefs of Staff.

Another group detected, called Fancy Bear, had targeted private and public sector networks around the world since the mid-2000s. The two groups involved in the DNC hacking had penetrated the system separately, CrowdStrike said.

Dmitri Alperovitch, CrowdStrike’s co-founder and chief technology officer, said the hackers specifically targeted the DNC’s research department and obtained opposition documents prepared about Trump. He said the firm is confident the DNC’s network has eliminated the threat.

But, Alperovitch said, “the Russians’ interest in the political campaign will not stop at this incident. We fully expect that they will try to get back in.”

A representative from the Russian Embassy in Washington did not immediately return phone calls and emails seeking comment Tuesday.

The incident was first reported Tuesday by The Washington Post.

“It should come as no surprise to anyone that political parties are high-profile targets for foreign intelligence gathering,” said Democratic Rep. Jim Langevin of Rhode Island, who co-founded the House’s cybersecurity panel. “Nonetheless, it is disconcerting that two independent operations were able to penetrate the DNC, one of which was able to stay embedded for nearly a year.”

Cybersecurity experts have previously told The Associated Press that neither Trump’s nor Hillary Clinton’s campaign networks are secure enough to stop attacks. Clinton’s use of a private email server while secretary of state raised questions of how well her personal system was protected from intrusions; her campaign has said there’s no evidence it was breached.