The home of a British diplomat was raided by police specialising in chemical and biological threats.
Reports suggest the house was searched over fears it was being used to hide chemical weapons.
Officers searched the home and garden of Nicholas Sutcliffe, a first secretary to the Foreign and Commonwealth Office (FCO), as well as the gardens of his neighbours.
Mr Sutcliffe, who has worked in Brazil and Cuba with the FCO, lives at the £650,000 four bedroom house in south London with his wife and four sons.
Reports suggest a 19-year-old man arrested under anti-terrorism laws following the raid and since released on bail, is one of Mr Sutcliffe's sons.
Neighbours told the Daily Mail more than 30 officers including a satellite truck descended on the house on Saturday morning, sealing off the road in Streatham Hill and putting up forensic tents over areas they had dug in the garden. They described them wearing protective clothing and carrying breathing equipment, which police said was a precautionary measure.
A spokesman for Scotland Yard said residents were at no “immediate risk” and said the operation had been pre-planned and led by intelligence.
They said: “The searches are being carried out as a precautionary measure and are ongoing.”
The teenager was arrested under the Anti-Terrorism, Crime and Security Act, which covers people suspected of terrorism as well as aiding or abetting the overseas use or development of chemical or biological weapons.
A 36-year-old woman was arrested at her home in Stratford in East London in a linked operation under the same legislation on Sunday afternoon.
Mr Sutcliffe has been a first secretary since 2001.
His brother, Peter Sutcliffe, 49, told the newspaper: “James and George are my nephews and they are 19-year-old twins. I find it extraordinary police have made an arrest and I cannot believe either James or George have involvement in terrorism.”