The U.S. Federal Bureau of Investigation says it has opened an investigation into whether the phones of victims of the 2001 terrorist attacks and their families were hacked into by media baron Rupert Murdoch's News Corporation.

Law enforcement sources said Thursday the FBI will look into whether employees at Murdoch's media firm illegally tried to access private calls, voicemail messages or call records of the victims and their relatives after the attacks nearly a decade ago, or looked to bribe police for such information. The FBI started the probe a day after Peter King, a congressman for the New York district where many of the 3,000 victims of the 9/11 attacks lived, asked the agency to investigate.

The FBI involvement is just the latest twist in the widening scandal engulfing Murdoch's vast media empire. In London on Thursday, the 80-year-old Murdoch and his son, James, at first refused, then agreed to testify before the British parliament next week about the phone-hacking and police bribery scandal that has engulfed their British media operations.

The elder Murdoch initially told parliament's media committee that he was not available to attend next Tuesday's hearing, while his son said he could not testify before August 10. But later in the day, after the committee summoned them to appear, the Murdochs changed their minds and said they will be there after all.

The Murdochs' agreement to testify came after the head of their British operations, Rebekah Brooks, a former editor of the now-closed News of the World tabloid at the center of the scandal, said she would appear before the panel. The Murdochs' decision to testify avoided a potential dispute over whether the parliamentary summons could be enforced against them because they are U.S. citizens — unlike Brooks, who is British.

The skirmishing over next week's hearing came on a day when British police arrested a ninth suspect in their investigation. He is Neil Wallis, the 60-year-old former executive editor of the News of the World who left the paper in 2009 and is now a public relations executive. Wallis was held on “suspicion of conspiring to intercept communications.”

Earlier in his career at the newspaper, Wallis was the deputy editor under Andy Coulson from 2003 to 2007. Coulson, the communications director for British Prime Minister David Cameron from 2007 until earlier this year, was arrested in the investigation earlier this month.

By the end of the day, police acknowledged that Wallis had been employed by them as a part-time consultant on a contract that ended last September. The French news agency said he worked for the police two days a month for a year and was paid $39,000.

The breadth of the scandal has rocked Murdoch operations in Britain, forcing the elder Murdoch to shut the 168-year-old News of the World last week and then abandon his $12-billion bid to acquire full control of British Sky Broadcasting, a satellite television company.

British politicians, including Mr. Cameron, have regularly sought to curry favor with the elder Murdoch.

But with the British public recoiling at the journalists' intrusion into the voicemails of everyday citizens, including those of murdered schoolgirl Milly Dowler, the British leader has turned against Murdoch. Mr. Cameron has named Lord Justice Brian Leveson to head a broad investigation into media operations in the country, as well as the specifics of what tactics Murdoch's journalists employed to gather information for their stories.

The fallout of the British scandal has jumped to the United States, where Murdoch owns the country's top business newspaper, The Wall Street Journal, and a major television news outlet, the Fox News Channel. Some U.S. lawmakers are calling for an investigation into allegations that Murdoch journalists attempted to hack into the voicemails of victims of the 2001 terrorist attacks in the U.S.

In Australia, Prime Minister Julia Gillard said Thursday that she may support a parliamentary review of the nation's media, following calls by Australian lawmakers to look into Mr. Murdoch's vast local media holdings.