A serving policeman has been arrested on suspicion of manslaughter, his force said.
The man, 34, was arrested after the death of a 64-year-old woman in Hillingdon, London, on Tuesday afternoon, the Metropolitan Police said.
His arrest, at his workplace, followed an "altercation" after which the woman died.
A Scotland Yard spokesman said: "The 34-year-old officer, who works in Central Operations, was arrested on suspicion of manslaughter this afternoon, Wednesday November 17, at his place of work and is currently in custody at a south London police station.
"The arrest follows an altercation that occurred in Snowden Avenue, Hillingdon, at approximately 4.30pm on Tuesday November 16, after which a 64-year-old woman died. Next of kin have been informed.
"The officer was off duty at the time. The woman was pronounced dead at the scene and post-mortem examination will take place tomorrow. Officers believe they know the identity of the deceased but await formal identification.
"Directorate of Professional Standards is aware and has informed the Independent Police Complaints Commission."
It is understood that the altercation involved motorists.
"I did not see anything, but a police officer knocked on the door and asked if she had seen a road rage accident," a resident of Snowden Avenue said.