Newly released testimony from former employees casts new doubt on the Murdochs' recent testimony before Parliament.
The two told lawmakers that the News of the World hacking scandal was less widespread than was being reported, and that they were unaware of it.
But former Murdoch employees said the two were not kept in the dark by subordinates and knew exactly what was going on.
"Nobody kept Mr. James Murdoch or any other News International/News Corporation executives from being in full possession of the facts," said ex-News International lawyer Jonathan Chapman in a letter to the House of Commons' media committee.
The attacks on the Murdochs' testimony are latest to pile the pressure on News Corp., which has already had to close the News of the World tabloid and scupper its multibillion dollar bid for satellite broadcaster BSkyB as the scandal rumbled on through the summer.
The controversy - which centers on allegations that reporters routinely listened to phone messages of public figures and bribed police officers to score scoops - has also claimed the jobs of Prime Minister David Cameron's top media aide, two top Scotland Yard officials and several long-serving newspaper journalists.
The Associated Press contributed to this report.