News International (NI) "closed the ranks" and refused to co-operate with Scotland Yard's original phone hacking investigation, the Leveson Inquiry heard today.
Peter Clarke, former head of the Metropolitan Police's counter-terror division, said he had a feeling the practice of illegally accessing voicemails was "endemic" in parts of the media.
However he defended his decision to restrict the scope of the investigation in late September 2006, despite evidence more journalists had carried out phone hacking and there could be many more victims.
The inquiry heard yesterday that police were obstructed, photographed and feared they might be attacked when they searched the News of the World's (NotW) offices for phone hacking evidence in August 2006.
Mr Clarke said today: "In terms of the investigation, it became immediately apparent that we weren't going to get any co-operation whatsoever from News International.
"Normally when one deals with something that's happening within a large international organisation or indeed a large national organisation, companies bend over backwards to try to preserve their reputation and assist with inquiries.
"This was a closing of the ranks from very early on."
NotW royal editor Clive Goodman and private investigator Glenn Mulcaire were jailed in January 2007 after they admitted intercepting voicemail messages left on royal aides' phones.
The retired Met assistant commissioner told the press standards inquiry he suspected that other journalists were also involved in hacking.
"If one journalist and a private investigator had found a means of doing this, it was probably inconceivable that more were not engaged, whether in that particular paper or wider," he said.
"I suppose it's a cynical view of an old police officer - if somebody is arrested for burglary, the chances of it being the first time they committed a burglary are slim."
Mr Clarke said he decided not to widen the hacking inquiry to cover other suspects and other victims in the light of the massive terrorist threat to the UK at that time.
Two days after the arrests of Goodman and Mulcaire on August 8 2006, police swooped on an extremist cell plotting suicide attacks on transatlantic airliners with the aim of killing hundreds of innocent people.
Mr Clarke told Lord Justice Leveson that counter-terror investigations outweighed "circumstantial" evidence in the phone hacking investigation.
"The circumstantial evidence against journalist A, B or C is a minor consideration in comparison with the consideration of what poses a threat to the lives of the British public," he said.