The News of the World hired former special forces soldiers to follow a police surveillance team tracking a suspect in the 2006 Ipswich prostitute murders, the Leveson Inquiry has heard.
Dave Harrison, a retired officer with the Serious Organised Crime Agency (Soca), said the newspaper jeopardised the official investigation into the killings.
He told the inquiry: "If our surveillance had been weakened by having to try and avoid other surveillance teams looking for us, if we had lost the subject, he may have gone and committed further murders because we were dealing with something else."
In December 2006, Suffolk Police asked Soca to provide surveillance officers to follow suspects in the then-unsolved murders of five women working as prostitutes in Ipswich, the hearing was told.
Mr Harrison said he and his colleagues were told at a briefing that the News of the World had employed its own surveillance team to identify who they were, where they were based and who the suspects were.
Asked how the paper learned that Soca officers were being sent to Ipswich, he said: "My opinion is it would have come from someone close to the investigation team, either the Suffolk inquiry or Soca."
Mr Harrison said that, on at least two occasions, a vehicle parked up on a roundabout on the outskirts of the town attempted to follow the Soca surveillance team. "We identified them because they were sat in the position that we would sit in if we were doing the same job," he said. "We were told that they were probably ex-Special Forces soldiers who would have a good knowledge of surveillance techniques."
The former Soca investigator criticised the actions of the News of the World, which was closed last July after the revelation that it hacked murdered schoolgirl Milly Dowler's phone.
He said: "Murder suspects before they are arrested, before they realise they are being investigated, may return to the scene of the crime. They may try to dispose of evidence, they may try to move bodies, they may even try to commit further offences...If they thought they were being followed, they might very well stop what they were doing or not do what they had planned to do."
Steve Wright was handed a whole-life sentence in February 2008 after being convicted of killing all five women.