A Libyan man has been arrested as suspect of the conspiracy to murder PC Yvonne Fletcher three decades ago.
PC Fletcher, 25, died after being shot while policing a demonstration outside the Libyan Embassy in London in 1984.
Scotland Yard described the arrest of the man, in his 50s, as a “significant development”.
He was detained on Thursday morning in south-east England and is now in custody. He is also suspected of money laundering offences.
Two other Libyan nationals – a woman in her 40s and a man in his 30s – were also arrested on Thursday on suspicion of money laundering.
Scotland Yard said the other two suspects, arrested at separate addresses in London and south-east England, are in police custody and that searches are under way across the country.
In a statement, PC Fletcher’s family said her father had recently died, with his “one regret in life” being that he had “never witnessed any justice” over the shooting.