Newly emergent China-linked threat operation Salt Typhoon has targeted major U.S. telecommunications providers AT&T, Lumen Technologies, and Verizon in attacks that reportedly involved the compromise of wiretap warrant requests in an effort to obtain intelligence, CNN reports.

Investigation into the incident, which was initially reported by The Wall Street Journal, has already been commenced by U.S. officials, who are worried about the implications of the breach on national security, according to sources close to the matter. Such an attack was also noted by the sources to have been raised to the House and Senate Intelligence committees, said the sources. Neither the reportedly impacted firms nor the Justice Department and the FBI commented on the report but the Chinese Embassy in Washington, DC, dismissed the reported hack as “a distortion of fact.” On the other hand, Microsoft, which was reportedly sought to investigate the attack, confirmed observing Salt Typhoon activity “consistent with public news reports.”