The so-called "plebgate" affair has gained momentum after two arrests were made in as many days and the Prime Minister demanded "proper answers" from the investigation.
A 46-year-old Metropolitan Police officer became the fourth to be arrested under the inquiry when she was held over alleged leaks to the media concerning the row that triggered former chief whip Andrew Mitchell's resignation.
A 46-year-old male colleague in the Diplomatic Protection Group (DPG) was bailed and suspended after he was arrested on Thursday on suspicion of the same offence - misconduct in a public office.
As Operation Alice picked up pace, David Cameron repeated his desire for a "thorough" investigation into the Downing Street altercation, during which it was claimed Mr Mitchell labelled serving officers "plebs".
Speaking in Liberia, where he is attending a UN meeting on international development, Mr Cameron said: "There is an ongoing police investigation, so it wouldn't be right for me to comment. All I would say is it is very important that this investigation is thoroughly carried out and proper answers are delivered."
The plebgate row ignited when Mr Mitchell was accused of a heated rant against officers as he left Downing Street on September 19.
Pressure intensified after The Daily Telegraph published a police log of the incident, which claimed he called officers "plebs" and swore at them repeatedly. He insisted he did not use the words attributed to him, and later said he was the victim of a deliberate attempt to "toxify" the Tories and ruin his career. An email from a civilian witness backing up the police account of events has also since been called into question.
Some 30 detectives have taken statements from all 800 officers in the DPG, which is tasked with protecting government officials. By January 16, Operation Alice, which has around 30 detectives working on it, had cost £82,500 since it was set up in December.
Two other men have been arrested so far, both of whom have been released on bail until February. A 52-year-old Pc, who is also from the DPG and currently suspended, was held on suspicion of misconduct in public office, while a 23-year-old was arrested on suspicion of intentionally encouraging or assisting the commission of an indictable offence.
Four other constables in the unit, which is responsible for protecting government officials and diplomats, have been placed on restricted duty over misconduct claims.