The committee could convene Senate hearings and subpoena News Corp witnesses and documents, although as yet there is no talk of doing so.

The commerce committee covers all means of communications - including telecommunications, free-to-air broadcasting and cable TV.

It also has oversight over the Federal Communications Commission, the regulatory body that has final say on the issuing of broadcast licences, including the 27 licences issued to Fox TV, the jewel in Murdoch's crown.

The FCC, which has come under pressure this week from ethics watchdogs calling for action against News Corp, can revoke licences if it deems that the companies holding them are not properly run in the public interest. In his letter, Senator Rockefeller asks whether some of the more than 5000 potential victims of phone hacking by the defunct News of the World may have been American.

''I am concerned about the possibility that some of these undisclosed victims are US citizens, and the possibility that telephone networks under the jurisdiction of US laws were used to intercept their voicemail messages,'' he wrote.