WASHINGTON — A key US senator called Tuesday for a probe into whether alleged hacking by Rupert Murdoch's media empire had extended to US citizens and warned of "severe" consequences if that proved the case.

"I encourage the appropriate agencies to investigate to ensure that Americans have not had their privacy violated," Senate Commerce Committee Chairman Jay Rockefeller said in a statement.

"I am concerned that the admitted phone hacking in London by the News Corp. may have extended to 9/11 victims or other Americans. If they did, the consequences will be severe," said Rockefeller, a Democrat.

His comments came as Murdoch faced an onslaught from British lawmakers as the government backed calls for him to drop his bid for pay TV giant BSkyB and a committee summoned him to answer questions on phone hacking.

Former prime minister Gordon Brown also piled pressure on Murdoch's media empire, accusing it of hiring "criminals" to obtain his private documents and suggesting it used illegal methods to break the news of his son's illness.

Scotland Yard added to the 80-year-old tycoon's woes, accusing his newspapers of blocking their investigations into hacking at the News of the World, the beleaguered tabloid that Murdoch axed on Sunday.

"The reported hacking by News Corporation newspapers against a range of individuals -- including children -- is offensive and a serious breach of journalistic ethics. This raises serious questions about whether the company has broken US law," said Rockefeller.

The lawmaker, who chaired the US Senate's intelligence committee for years, was the first major voice in the US Congress to call for an investigation into the scandal, which has captivated Britain.

Murdoch flew to Britain Sunday to take control of the crisis and met Tuesday with News International chief executive Rebekah Brooks, who was News of the World editor at the time of some of the hacking, and other key figures.

Lawmakers took advantage of his presence to call on him, his son, News Corp. executive James Murdoch, and Brooks to appear to face questions about hacking and allegations that Murdoch papers paid police for information.

News International said it would "cooperate" with the request from the House of Commons culture, media and sport select committee to appear next week, but did not immediately confirm their attendance.

Murdoch was not the only one facing pressure from lawmakers.

Senior police officers were grilled by a committee on Tuesday about how an original probe into the News of the World in 2006, which resulted in the jailing of two people, failed to unearth a trove of further allegations.

The fresh claims, including that the tabloid hacked the voicemails of a murdered teenager and the relatives of dead soldiers, finally emerged last week and unleashed the public outrage that led to the demise of the paper.

Scotland Yard Assistant Commissioner John Yates, who decided not to reopen the investigation in 2009, apologized to the victims but blamed News International for failing to hand over key evidence.