London (CNN) -- More than two dozen News International employees used the services of a convicted phone-hacker, the British government-backed inquiry into illegal eavesdropping and bribery by journalists heard Monday.
"This fact alone suggests wide-ranging, illegal activity within the organization at the relevant time," lawyer Robert Jay said Monday.
Prime Minister David Cameron established the Leveson Inquiry after public outrage at the revelation that the tabloid newspaper News of the World illegally hacked into the phone messages of a 13-year-old murder victim.
The missing girl, Milly Dowler, was among up to 5,800 crime victims, royals, politicians and celebrities alleged to have been targeted by the best-selling newspaper in search of stories. The tabloid was shut down in July because of the scandal.
The paper's publisher, News International, and its chief executive, James Murdoch, have always insisted that the practice of phone hacking was not widespread.
But Jay, one of the lawyers serving as part of the Leveson Inquiry, said Monday that private investigator Glenn Mulcaire's notebooks contained the names of at least 28 people who employed him on 2,266 occasions.
Four individuals -- whom Jay did not name -- were responsible for almost all the Mulcaire commissions, Jay said.
But the sheer number of names in Mulcaire's files means the one News of the World journalist jailed over phone hacking, Clive Goodman, "was not a rogue reporter," Jay said.
Mulcaire and Goodman went to prison in 2007 after admitting hacking into royal family staff messages.
News International did not immediately respond to a CNN request for comment on Jay's allegations Monday.
The Leveson Inquiry will hear from celebrities including Hugh Grant and J.K. Rowling, who believe they may have been victims of phone hacking, as well as from journalists, crime victims, politicians and other experts.
Actor Jude Law is suing another News International paper, The Sun, because he believes he was hacked.
Jay said there is one reference to The Sun in Mulcaire's notebooks, as well as another one which may refer to the Daily Mirror, a tabloid published by a rival newspaper group.
Most British newspaper groups, including News of the World publisher News International, got "core participant" status in the investigation, as did its former editor, Rebekah Brooks.
So did the Metropolitan Police, which has been accused of bungling the original investigation into phone hacking. Officers are also accused of taking bribes from journalists.
Lawyers for core participants can examine witnesses at the inquiry.
Police are investigating both the phone-hacking and the bribery allegations, and lawmakers are conducting their own separate probes.
Last week, a parliamentary committee investigating the hacking grilled James Murdoch.
The son of media tycoon Rupert Murdoch rejected allegations Thursday that his company behaved like a Mafia organization over the scandal.
Questioning Murdoch about why so little information was shared, Labour Member of Parliament Tom Watson suggested the company operated under an "omerta" code of silence.
He said an "omerta" involves "a group of people who are bound together by secrecy, who together pursue their group's business objectives with no regard for the law, using intimidation, corruption and general criminality."
"Would you agree with me that this is an accurate description of News International in the UK?" Watson asked.
Murdoch replied: "Absolutely not. I frankly think that's offensive and it's not true."
To gasps of amazement from fellow members of the Culture, Media and Sport Committee, Watson went on: "You must be the first Mafia boss in history who didn't know he was running a criminal enterprise."
Murdoch rolled his eyes before saying that comment was "inappropriate."
He said he "disputed vigorously" the version of events described by former News of the World editor Colin Myler and its former legal manager, Tom Crone, who both left the company when the tabloid was closed.
Myler and Crone claimed they made Murdoch aware of the contents of an e-mail in 2008, indicating phone hacking was widespread. The document is known as the "for Neville" e-mail, apparently after its intended recipient, Neville Thurlbeck, then News of the World's chief reporter.
Analysts said Murdoch was caught in a "Catch-22" situation: if he wasn't aware of the extent of phone hacking when the company paid an out-of-court settlement of $1.2 million to one victim, he could be seen to be incompetent; if he did know about it, it could be an indication there was a cover-up.
Murdoch has maintained he found out about the full extent of phone hacking by staff and investigators at News of the World in 2010 and was not shown the e-mail.
CNN's Jonathan Wald, Erin McLaughlin, Peter Wilkinson, Atika Shubert and Laura Smith-Spark contributed to this report.