U.S. officials suspect that a recently discovered Chinese hacking and espionage campaign scooped up data on hundreds of thousands of American mobile phone users, likely stealing information about more than 1 million customers, ABC News has learned.

Sources familiar with the investigation gave ABC News a detailed sense of the vast scope of what U.S. officials are calling a major intelligence-gathering operation by China, exploiting weaknesses in the communications networks of the nation’s top telecommunications companies.

In a briefing on Tuesday, FBI and Homeland Security’s cyber officials said that the Chinese were able to steal a “large amount of data” — officially acknowledging for the first time that China was able to determine who thousands of mobile phone users were talking to, when they spoke and where they were communicating from.

China initially appeared to have focused on a huge swath of mobile phone users in the national capital region. The people who briefed ABC News declined to provide numbers, but ABC News was able get more detailed information through a number of interviews.

The Chinese hackers were then able to narrow their focus and zero in on the specific communications of a smaller number of important, high-profile Americans and, in some cases, China was able to obtain audio calls from that smaller group of victims and review their text messages.

ABC News has learned that among those targeted were some top government officials in the Biden administration, with sources telling ABC News this includes at least one cabinet secretary and a top White House Homeland Security Adviser.

Although it is not clear how deep the spying went on specific targets, sources tell ABC News that a number of those high-ranking government officials and high-profile Americans have been informed that they were potential victims.

ABC News has previously reported that President-elect Donald Trump, Vice President-elect JD Vance, some staff of Sen. Majority Leader Chuck Schumer, one of Trump’s defense attorneys, Todd Blanche, and leaders in both political campaigns were targeted.

Officials in the Tuesday briefing admitted they cannot say with certainty that China hackers have been fully kicked out of these telecommunications networks and acknowledged they are still trying to understand the scope of this activity.

Multiple sources have warned that the scope and scale of the operation continues to widen, with one official telling ABC News that this is not over.

The investigation has revealed that China’s campaign exploited U.S. computer routers serving telecom corporations, giving them the gateway to the phone numbers of significant numbers of customers of Verizon, AT&T, Lumen Technologies and other telecommunications companies.

ABC News previously reported that the operation had gone undetected for many months, and was perhaps carried on for more than a year.

A recent statement by FBI and Homeland Security officials also revealed that China also got access to some data about some sensitive warrants the Department of Justice has been pursuing, with sources telling ABC News that there is a fear of China using the data to figure out some of the people who federal authorities are investigating and, in some cases, monitoring.

Much of the Tuesday briefing centered on how Chinese hackers compromised the system by exploiting existing basic gaps in security and confirmed that federal authorities have given industry leaders a list of remedies to detect and prevent ongoing attacks as well as recommendations on how to root the hackers out.