The FBI is investigating reports that Rupert Murdoch's News Corporation sought to hack the phones of victims of the 9/11 attacks.
The criminal probe follows calls from a growing number of senators and a senior Republican for an investigation.
In an interview with the Wall Street Journal, Mr Murdoch defended the company's handling of the crisis.
Meanwhile, Mr Murdoch and his son James have agreed to answer UK MPs' questions on the hacking scandal next week.
The Commons media committee issued summonses after the pair initially declined to appear.
Also in London on Thursday, a former News of the World executive editor, Neil Wallis, became the ninth person involved with the newspaper to be detained by police probing phone hacking.
The Murdoch-owned Sunday tabloid was shut down last week amid the mounting scandal over the alleged hacking of phones belonging to crime victims, politicians and celebrities.
Continue reading the main story Analysis Steve Kingstone BBC News, Washington
Unquestionably, the hacking scandal has crossed the Atlantic.
The FBI's involvement takes the affair beyond the rumblings of US politicians into the much more dangerous territory of a criminal investigation.
Of course, it remains to be proved whether Americans' phones were hacked.
The allegation that 9/11 victims were targeted by the News of the World was made by a rival British newspaper, the Daily Mirror, which based its report on an unnamed source.
If evidence of US hacking is uncovered, the scandal will take on a whole new dimension - especially if it touches upon the emotive subject of 9/11.
The company could face a second line of enquiry, into whether payments to British police officers breached US anti-corruption laws.
FBI sources told US media on Thursday it was looking into claims that phones belonging to victims of the September 11 attacks could have been hacked by News of the World journalists.
News Corporation, based in New York, is the parent company of News International, the UK firm at the centre of the scandal over phone hacking and payments to police officers.
The fallout prompted News Corporation on Wednesday to withdraw from a huge takeover bid for the UK's largest pay-TV operator, BSkyB.
UK Prime Minister David Cameron has set up a judge-led inquiry into the allegations.
US senators this week asked the authorities to investigate allegations that 9/11 victims' phones were hacked. News International has not commented on the claims.
Democratic senators Jay Rockefeller and Barbara Boxer urged the attorney general and the Securities and Exchange Commission to investigate whether US laws had been broken.
Democratic Senator Robert Menendez, who wrote to the attorney general separately, said claims that newspapers had sought to "exploit information about... personal tragedies for profit" needed to be investigated.
Prince Al-Waleed bin Talal Alsaud: Brooks should resign if there is any sign of her involvement
Republican Congressman Peter King - who is chairman of the House homeland security committee and represents a constituency in New York that lost more then 150 people in the 9/11 attacks - called on Wednesday for an FBI inquiry.
"The thought that anyone would have hacked into the phones of either those who were killed, those who were missing, the family members, during that tragic time... is contemptible," he told the BBC on Thursday.
He declined to say whether he had any direct evidence that journalists working for News Corporation companies had sought to hack 9/11 victims' phones.
Prince backs Murdoch
"There were allegations, I want the FBI to investigate them, I'm not making any conclusions at all," he said.
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Source: Columbia Journalism Review
In an interview with the News Corporation-owned Wall Street Journal released on Thursday, Mr Murdoch said his company had handled the crisis "extremely well in every way possible", but acknowledged "minor mistakes".
Asked for his reaction to recent negative media coverage of News Corporation, Mr Murdoch said he was "just getting annoyed. I'll get over it. I'm tired".
There was some good news for Mr Murdoch on Thursday.
Saudi Prince Alwaleed bin Talal, one of News Corporation's largest shareholders, told the BBC he had no plans to sell off his stake in the company - 7% of voting shares.
But the Saudi royal, a nephew of the Saudi Arabian king, said he thought News International Chief Executive Rebekah Brooks should quit.
"For sure she has to go, you bet she has to go," he told the BBC's Newsnight programme.