The FBI has opened an investigation into allegations that News Corporation sought to hack into the phones of 11 September victims.
The news came as American politicians called for a US investigation to be mounted into the phone-hacking scandal.
The FBI's New York office last night confirmed the investigation was under way. There has been no response from News Corp.
A key member of Congress's investigations committee joined the clamour for a US-based inquiry into the allegations that News Corp staff broke American anti-bribery laws or violated the privacy of 9/11 victims.
Bruce Baley, the vice-chairman of the investigations and oversight committee, said that Congress had "important oversight responsibilities" to get "to the bottom of this evolving scandal".
The fall-out from the controversy crossed the Atlantic with a vengeance as Congressman Baley suggested that News Corp may have engaged in "political espionage or personal espionage".
Mr Baley was speaking after other US lawmakers urged federal officials to find out if News Corp broke a law banning bribes to foreign officials. The call was made in response to accusations that News of the World journalists paid cash to police officers.
The US Foreign Corrupt Practices Act (FCPA) prohibits payments intended to influence any act or decision of a foreign official. Democratic senators Barbara Boxer, Jay Rockefeller and Frank Lautenberg on Wednesday asked Attorney General Eric Holder and Securities and Exchange Commission Chairman Mary Schapiro to look into potential FCPA violations.
"The reported allegations against News Corp are very serious and indicate potentially thousands of victims and a pattern of illegal activity. It is important to ensure that no US laws were broken and no US citizens were victimised," Mr Rockefeller and Mr Boxer wrote in a letter sent to the Attorney General and the Securities and Exchange Commission.
Mr Lautenberg claimed that, as a US-based company, News Corp and its employees are subject to the anti-bribery provisions of the FCPA.
He said: "The limited information already reported in this case raises serious questions about the legality of the conduct of News Corporation and its subsidiaries under the FCPA."
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US interest in the hacking scandal was given fresh momentum by the suggestion that 9/11 victims' phones were hacked.
The suggestion was based on a Daily Mirror report claiming that a former New York police officer turned private investigator was alleged to have been contacted by News of the World journalists. It was claimed that they offered him money to retrieve the phone numbers of the dead and details of the calls that they had made and received in the days before the attack on the Twin Towers. News International has said there is "no evidence" to support this allegation.
But last night Melanie Sloan, of Citizens for Responsibility and Ethics in Washington, said: "There is some evidence to suggest that the News of the World attempted to hack into the phones of 9/11 victims."
If it was proved that the phones of 9/11 victims had been hacked, Ms Sloan said it would be a "huge scandal" in America.
"We need to ask questions here in the US whether the same tactics have been used in the US and whether British journalists used these tactics against Americans," she added.
Coverage of the phone-hacking story by Murdoch-owned media outlets in the US has provoked much comment from observers. Although Fox News, the enormous Murdoch television enterprise, is now reporting on the scandal, there were eyebrows raised when there was no mention of News Corp's decision to withdraw its bid for BSkyB when the news broke.
In contrast, the Wall Street Journal, one of the most revered of the Murdoch titles, has been reporting the story.
In Rupert Murdoch's native Australia there was also a backlash. His company News Limited has announced an audit of expenditure by its Australian newspapers in a bid to ensure their journalistic practices are clean.The head of Mr Murdoch's Australian division, John Hartigan, said: "We will be conducting a thorough review of all editorial expenditure over the past three years to confirm payments to contributors and other third parties were for legitimate services."
Mr Hartigan has previously said he believes the group's Australian outlets, which include the Herald Sun in Melbourne and The Australian, have not engaged in the types of practices that forced the closure of the News of the World.