The US is in the early stages of an investigation into potential Chinese hacking of American telecommunications companies, according to a top intelligence official.

The director of the National Security Agency said Sunday that a Chinese hack of three major telecommunications firms is under investigation but that it remains “premature” to discuss the case.

A highly skilled group of Chinese government-linked hackers has in the last several months infiltrated multiple US telecommunications firms in a likely search for sensitive information bearing on national security,

This includes AT&T, Verizon, and Lumen. Investigators suspect the hackers were looking for national security or other sensitive information.

A group of hackers with backing from the Chinese government broke past the security of multiple US telecom firms, including AT&T and Verizon, and potentially accessed data used by law enforcement officials.