Under pressure, Cameron cut short a trip to Africa and returned home to appear at Britain's Parliament.
Lawmakers want to know why he insisted on hiring his former director of communications, Andy Coulson.
Coulson has been arrested in connection to the scandal. He was an editor at the now shut down News of the World tabloid. Cameron defended his decision to hire Coulson, saying he believes that people are innocent until proven guilty.
"If it turns out that he knew about the hacking, then that will be a matter of huge regret, a matter of great apology, a disgrace not only that he worked in government, but also vitally something that would be subject to criminal prosecutions," said Cameron.
On Tuesday, Rupert Murdoch and his son, James, faced three hours of tense questions.
James Murdoch apologized to the victims. His father rejected blame for the scandal, insisting he was only at fault for trusting the wrong people.
"It's a matter of great regret of mine, my father's and everyone at News Corporation," said James Murdoch. "These actions do not live up to the standards that our company aspires to everywhere around the world."
The most dramatic moment in the hearing didn't have anything to do with testimony. A man aimed a plate of shaving foam at Rupert Murdoch.
A News Corp. attorney partially blocked the attack and Murdoch's wife, Wendi Deng, slapped the man. He was arrested and charged.
Murdoch wasn't hurt. In fact, the 80-year-old billionaire simply took off his splattered suit jacket and continued answering questions.
The former head of Murdoch's news operation in Britain appeared after him. Over the weekend, Rebekah Brooks was arrested, questioned and released.
Brooks said she had been told by News of the World journalists that allegations of phone hacking were untrue and that she never knowingly sanctioned a payoff to a police officer.
Meantime, a British judge has awarded actor Hugh Grant the right to see evidence that could reveal whether his voicemails were intercepted by News of the World.
The judge said police should disclose information to him that was allegedly gathered by a private investigator working for the paper.
Grant has been one of the loudest celebrity critics since the scandal broke.