Under cross examination at London's Old Bailey, Brooks was asked to explain why News International, News Corp.'s British arm, had publicly stated phone-hacking was limited to a "rogue" reporter from the News of the World jailed in 2007 until that position no longer became tenable two years later.
Brooks denies conspiracy to hack phones, authorising illegal payments to public officials and conspiracy to pervert the course of justice.
Asked whether she believed the company's behaviour was honourable during this period, Brooks replied: "At the time I did - I had no reason to believe otherwise."
Prosecutor Andrew Edis asked her if she had carried on with the "cover-up" when she became chief executive in 2009, she replied: "No".
Private detective Glenn Mulcaire and the News of the World's former royal editor Clive Goodman were jailed in 2007 after pleading guilty to phone-hacking. Both admitted hacking phones of members of Britain's royal household and Mulcaire also said he had tapped the phone messages of five other well-known figures.
Brooks, in the witness box for a ninth day, was asked to explain how her ignorance about the extent of phone-hacking at the News of the World fitted in with earlier testimony she gave in which she said police had told her in 2006 that there had been 100 to 110 victims.
"I think my state of mind at the time was that Mulcaire had worked for others at the News of World as a private detective," she told the jury.
"The suggestion I got from police was they had no evidence to suggest this went wider at the News of the World."
Her trial and that of six other continues.