Two groups of hackers sponsored by the Russian government broke into the computer systems of the Democratic National Committee and accessed emails, chat logs and a trove of detailed opposition research on Donald Trump assembled by the party’s researchers.

The hackers were removed from the system over the weekend, officials told The Washington Post, but not before they were able to comb through staff members’ day-to-day communications and their research on the presumptive Republican nominee.

One hacking group entered the system last year, and been monitoring DNC communications since then, The Post’s Ellen Nakashima reported. This April, the other group gained access to the Trump files.

Paired with the ongoing controversy over the private email server Hillary Clinton used during her time as secretary of state, the enormous breach at the DNC is the second large-scale cybersecurity setback that Democrats have run into during the presidential campaign.

The persistence and skill of the Russian hackers that penetrated the DNC’s defenses illustrates the threat posed by foreign online espionage. In the past, Russian hackers have gained access to email systems at the White House and the State Department, and their Chinese counterparts pulled off a colossal heist at the Office of Personnel Management last year, stealing the sensitive personal information of more than 22 million Americans.

The discovery of the Russian espionage operation vindicated James Clapper, the director of national intelligence, who warned last month that foreign intelligence agencies were spying on U.S. presidential campaigns. Indeed, as far back as 2008, the intelligence community gave a presentation to then president-elect Obama and his advisers, warning them that foreign intelligence services “have been tracking this election cycle like no other.”

Russian spies appeared to have targeted more than just the DNC during this election. Officials told The Post that hackers also went after Clinton and Trump’s campaigns, as well as several Republican political action committees.

The hackers appeared to target internal communications and opposition research only, leaving untouched the DNC’s vast stores of personal and financial information on donors. That suggests sophisticated political espionage, rather than an opportunistic criminal lunge for potentially lucrative data.

The ease with which the hackers were able to listen in on the DNC’s communications is a far cry from the last major attempt to eavesdrop on the Democratic Party, more than 40 years ago. Then, malfunctioning wiretaps on DNC phones led officials to approve a plan to break into the party’s Watergate headquarters, a plot which eventually led to Richard Nixon’s resignation.

This year, the Russians’ electronic intrusion raised alarms only in April, when a network administrator saw suspicious activity and alerted party leaders. (It’s much harder to find hackers in a computer system than it is to spot a taped-up door.)

World leaders have closely tracked the U.S. presidential campaign as it has developed over the past 12 months, especially Russian President Vladimir Putin, who has expressed admiration for Trump’s brash style. Trump, in turn, praised Putin’s leadership.

It’s not unusual for governments to monitor other countries’ political campaigns in order to glean details on a candidate’s policies before he or she is elected to high office. Because American intelligence agencies are known to spy on foreign governments, it’s likely that they are also listening in on crucial political races overseas. But in a presidential campaign where the discussion of cybersecurity hasn’t moved past a political tussle over Clinton’s email server, the Russian intrusion is a reminder of the prevalence of the threat—and the expertise of the U.S. adversaries.