The National Security Agency is investigating a hack that involves three major telecommunications firms — AT&T, Verizon and Lumen Technologies.

David Hyde, a professor of computer science at Vanderbilt University, said that based on the available information, some of the nation’s largest telecommunications providers appear to have suffered a breach “believed to be associated with Chinese state-sponsored hacking groups.”

According to a source with knowledge of the matter, the hackers may have been trying to access systems the federal government uses for court-approved electronic surveillance.

“It’s particularly bad that the traffic these attackers would have been able to see would have some strong correlation in principle with activities of the U.S. intelligence and law enforcement communities,” said Hyde.

Milos Manic, director of the cybersecurity program at Virginia Commonwealth University, said that telecommunications companies are usually prime targets for hackers because of the information being stored.

“Companies like that are target-rich because they have massive amounts of very sensitive data,” Manic said.

As for what was potentially stolen, Manic said he’d rather not speculate as right now, not much is being shared.

“It’s not in the interest of national security to publicize the details,” said Manic.

A spokesperson from China’s embassy responded to the allegations, saying in a statement that the U.S. intelligence community and cybersecurity companies are spreading disinformation.