The deputy chief constable of Surrey Police is being investigated over claims that he failed to act over the hacking of Walton-on-Thames schoolgirl Milly Dowler's phone.
Craig Denholm, who was the senior investigating officer on the Dowler case in 2002, is being investigated along with temporary Detective Superintendent Maria Woodall, who took over as senior investigating officer in 2006, the Independent Police Complaints Commission (IPCC) said.
The investigation will focus on the senior officers' alleged knowledge that Milly's phone was hacked by the News of the World in 2002 and any actions taken, the watchdog said.
Mark Lewis, the Dowler family's lawyer, said: "The Dowler family welcomes the proper investigation of what happened at Surrey Police 10 years ago.
"They regret that the passage of time means that some individuals can now no longer be investigated."
An IPCC spokesman said: "The IPCC investigation is considering whether Mr Denholm was aware during Operation Ruby that the NOTW (News of the World) had accessed Milly Dowler's voicemail in 2002 and his handling of that information."
He added that the watchdog was also examining information provided by Woodall to Surrey Police during the internal inquiry into the force response to allegations that Milly's voicemail had been illegally accessed in 2002.
Neither officer has been suspended and both remain on duty, a joint statement by the force and the police authority said.
The two referrals to the police watchdog come after it found in February that there was no evidence to support allegations that a Surrey police officer gave information to journalists during Operation Ruby, the investigation into Milly's disappearance in 2002.
The now defunct News of the World admitted hacking the 13-year-old's mobile phone but it remains unknown whether two missing messages were deleted deliberately, as previously suggested, or were removed from her message box automatically.