Rupert Murdoch's global media empire is facing a challenge on a new front in the billowing phone-hacking scandal after a powerful US Senate committee opened direct contact with British investigators in an attempt to find out whether News Corporation has broken American laws.

Jay Rockefeller, chairman of the Senate committee on commerce, science and transportation, has written to Lord Justice Leveson, who leads the British judicial inquiry into media ethics, asking if he has uncovered any evidence relating questionable practices in the US.

"I would like to know whether any of the evidence you are reviewing suggests that these unethical and sometimes illegal business practices occurred in the United States or involved US citizens," Rockefeller writes in a letter released on Wednesday.

The development adds to the potential dangers facing News Corp, a publicly-traded company with its headquarters in New York. Rockefeller has taken a close interest in the unfolding phone-hacking saga, but it is the first time that a Senate committee member has acted in his official capacity.

Should the committee decide to press its case, it has considerable powers at its disposal. It could convene official Senate hearings into the scandal and subpoena witnesses and documents from News Corp – though as yet there is no discussion of doing so.

The commerce committee covers all means of communications in the US – 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 the Fox TV network that is 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.

John 'Jay' Rockefeller. Photograph: AP

In his letter to Leveson, Rockefeller asks whether some of the more than 5,000 potential victims of phone hacking by the now-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 adds that he wants to know whether any News Corp business had "used hacking, bribing, or other similar tactics when operating in the US".

In a scathing attack on the Murdoch company, Rockefeller writes: "In a democratic society, members of the media have the freedom to aggressively probe their government's activities and expose wrongdoing. But, like all other citizens, they also have a duty to obey the law.

"Evidence that is already in the public record clearly shows that for many years, News International had a widespread, institutional disregard for these laws."

Rockefeller also asks for details emerging from the Leveson inquiry that indicated whether any News Corp executives based in New York were aware of illegal payments made by News of the World to British police and other public officials. "I would be very concerned if evidence emerged suggesting that News Corporation officials in New York were also aware of these illegal payments and did not act to stop them."

The senator does not name individuals. Rupert Murdoch and his son James – who was until recently the chairman of News Corp's British newspaper division, News International – are based in the New York headquarters, as was Les Hinton, James Murdoch's predecessor, who was this week accused by the UK Commons culture committee of misleading parliament. Hinton resigned in July.

Rockefeller's intervention was triggered by the final report of the British parliament's culture, media and sport select committee, which concluded that Murdoch was not fit to run a major international company. It comes two weeks after Mark Lewis, a British lawyer at the forefront of the phone-hacking investigations, opened investigations into four cases of alleged phone hacking that occurred in the US.

Lewis, who won £3m for the family of murdered teenager Milly Dowler from News International, said: "There are so many American aspects to this, it is difficult to know where to begin. Whether it is about American victims of hacking, or the failure of a US company in respect of the corrupt practices of a foreign subsidiary, or the fact that material information was witheld from shareholders and potential investors: I suspect it is a race to find out which scandal lands first.

"It was only a matter of time before the US authorities realised the significance of what had been happening in the UK."

He is set to launch six cases in the US with Norman Siegel, an American lawyer who has previously represented families of September 11 victims. He has not divulged who his clients are, but it is known one is actress Koo Stark, who has been told by Metropolitan police detectives that she was targeted by the hacking operation run by the News of the World.

On Wednesday, the News Corp board issued a statement backing Rupert Murdoch, saying its members had "full confidence" in his" fitness" to run the company he built from a single newspaper in Adelaide.

News Corporation said that it based its vote of confidence on "Rupert Murdoch's vision and leadership" in building the company from its modest roots, "his ongoing performance", and his "demonstrated resolve to address the mistakes of the company identified in the select committee's report."

On the same day as the Senate commerce committee made its move, a second US senator, Frank Lautenberg of New Jersey, renewed his call for a US government investigation into whether News Corp broke anti-bribery and corruption laws. Lautenberg called for a robust inquiry into whether the company, by allegedly bribing public officials in he UK, had breached the Foreign Corrupt Practices Act, which forbids American citizens or companies from engaging in acts of bribery abroad.