London (CNN) -- Two senior police officers are being investigated over their alleged knowledge that the phone belonging to murdered schoolgirl Milly Dowler was hacked, the UK's independent police watchdog said Thursday.
The revelation last summer that the missing 13-year-old's voicemail was hacked by the News of the World newspaper in 2002 led to the closure of the paper and sent shockwaves through the British establishment.
Deputy Chief Constable Craig Denholm of Surrey Police is one of those being investigated "in relation to his alleged knowledge that Milly Dowler's mobile phone was illegally accessed by the News of the World," the Independent Police Complaints Commission said.
The Surrey Police Authority voluntarily referred his conduct to the watchdog a week ago, the IPCC said in a statement.
Denholm, then a detective chief superintendent, was the senior investigating officer for part of Operation Ruby, the Surrey Police investigation into the abduction and murder of Milly Dowler in 2002, it said.
The inquiry will look at whether he was aware at the time that the Murdoch-owned newspaper had accessed Milly Dowler's voicemail in 2002 and his handling of that information.
The watchdog is also investigating the conduct of temporary Detective Superintendent Maria Woodall, who was the senior investigating officer on Operation Ruby from 2006, it said.
Its focus will be "the information she provided to Surrey Police during the course of the internal inquiry into the force response to allegations that Milly Dowler's voicemail had been illegally accessed in 2002."
The family of Milly Dowler is aware of the investigation, the watchdog 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," said a statement released on the family's behalf.
The Met Police continues to investigate claims of phone hacking, known as Operation Weeting.
A parallel police operation is investigating claims of inappropriate payments to police and public officials.
A 31-year-old man on Thursday became the latest to be arrested in connection with the latter operation, police said.
The man, a former National Health Service worker, was detained at his home in Uxbridge, west of London, on suspicion of conspiracy to cause misconduct in a public office and on suspicion of corruption, a police statement said.
Prime Minister David Cameron established the Leveson Inquiry, an idependent judge-led inquiry into media ethics, amid wide public anger at the News of the World about the hacking of Milly Dowler's voice messages.
Cameron and other senior present and ex-government figures have been called to testify before the inquiry, as have News Corp. media baron Rupert Murdoch and his former UK deputy, Rebekah Brooks.
Milly Dowler's parents told the inquiry last November how phone hacking on behalf of News of the World had given them false hope their missing daughter was still alive.
In fact, the messages had been deleted by a private investigator working for News of the World, Dowler's father, Bob, told the inquiry panel. Milly Dowler had already been murdered.