More than 100 new allegations of "data intrusion" also are being probed.
Deputy Assistant Commissioner Sue Akers' comments to a judge-led inquiry into media ethics indicated that the scandal, which erupted last year at Rupert Murdoch's News of the World and has involved hundreds of victims, could end up burning the now-defunct tabloid's U.K. competitors as well.
Akers gave as an example payments of tens of thousands of pounds (dollars) allegedly made to the same prison officer by all three newspaper groups.
"Our assessment is that there are reasonable grounds to suspect offences have been committed and that the majority of these stories reveal very limited material of genuine public interest," Akers told Lord Justice Brian Leveson, who is leading a government inquiry into media misbehaviour set up in the wake of the scandal.
Separately, prosecutors said they would announce Tuesday whether to levy criminal charges against an unspecified number of people caught up in the investigation.
So far more than 40 journalists and public officials have been arrested as part of the sprawling inquiry. Only a handful, including former News International chief executive Rebekah Brooks, have been charged. Brooks has denied any wrongdoing
In her testimony, Akers also said her force was combing through a mountain of electronic information to find evidence for more than 100 claims of what she called "data intrusion" — a category which includes computer hacking and improper access to medical records.
In what might be a newly discovered tabloid espionage technique, she said that in at least two cases detectives had discovered data which "appears to come from stolen mobile telephones."
Police were examining "whether these are just isolated incidents or just the tip of the iceberg," Akers said.
The phone hacking scandal erupted last July after it emerged that journalists at the News of the World routinely eavesdropped on cellphones' voicemail boxes in order to score scoops. The probe has since grown to take in allegations of computer hacking and bribe-paying across Murdoch's News International — and beyond.
Several calls to Express Newspapers, owned by Richard Desmond's Northern and Shell PLC, weren't immediately returned. In an email statement, Trinity Mirror spokesman Nick Fullagar said that "we take any accusation against the company very seriously and we are co-operating with the police on this matter," noting that the newspaper group remained engaged with Leveson's inquiry.
He added: "This is all we are saying."
Police have been widely criticized for their failure to come to grips with the hacking issue when it first emerged nearly seven years ago. Police repeatedly ignored crucial leads and dismissed new evidence, claiming that phone hacking was a limited practice affecting only a handful of people.
On Monday, Akers gave the force's most up-to-date accounting yet, telling the inquiry that more than 702 people "are likely to be victims."