CNN  — 

The Chinese hackers who breached multiple US government agencies in May stole some 60,000 emails from senior State Department officials, including information on officials’ travel itineraries, a Senate staffer briefed on the matter told CNN.

The new details from a briefing to senators and their staffers illustrate how the Chinese operatives allegedly scoured the inboxes of senior US diplomats focused on diplomacy in the Pacific ahead of a high-stakes trip to China by Secretary of State Antony Blinken in June.

The hackers raided the unclassified email accounts of nine State Department officials focused on East Asia and the Pacific, and another official who works on European issues, according to a staffer in the office of Republican Sen. Eric Schmitt of Missouri. The staffer, who declined to be named, attended a briefing on the hacking campaign that senior State Department IT officials gave the Senate on Wednesday.

The hackers were also able to access a list of every State Department email address, according to the Senate staffer. That kind of reconnaissance could be useful information for any follow-on hacking efforts aimed at the State Department.

At a press briefing Thursday, State Department spokesperson Matthew Miller confirmed that the hackers accessed about 60,000 State Department emails.

“[T]his was a hack of Microsoft systems that the State Department uncovered and notified Microsoft about,” Miller told reporters. The State Department has not formally attributed the hack, but Microsoft has blamed a “China-based” hacking group. “We have no reason to doubt [Microsoft’s] attribution in this case,” Miller said.

The stealthy hacking campaign exploited Microsoft email software and began with the hackers breaching a Microsoft engineer, the company has said. The cyber intrusions have showcased the leaps China has made in its cyber capabilities, according to experts, and have prompted US lawmakers and Biden administration officials to scrutinize the US government’s reliance on Microsoft technology.

The cyber-espionage campaign breached the unclassified email accounts of US Ambassador to China Nicholas Burns, Commerce Secretary Gina Raimondo, and Daniel Kritenbrink, the assistant secretary of State for East Asia who traveled with Blinken to China in June, CNN previously reported.

Republican Rep. Don Bacon of Nebraska, who has been critical of the Chinese government, said he was also breached by the hackers.

The State Department declined to comment on the Senate briefing.

Chinese government officials have responded to the hacking allegations by accusing the US government of conducting cyberattacks against China.

In a statement to CNN, Schmitt welcomed the briefing but said his investigation into the hacks is “far from over.”

“We need to harden our defenses against these types of cyberattacks and intrusions in the future, and we need to take a hard look at the federal government’s reliance on a single vendor as a potential weak point,” Schmitt said.

CNN’s Alex Marquardt and Michael Conte contributed reporting.