A 61-year-old man is being questioned after he was arrested at London's Olympic Park site on suspicion of supplying an explosive substance.
He is the third suspect to be arrested by the Metropolitan Police after officers from its Olympic Site Support Unit stopped and searched a woman in a car park off Pudding Mill Lane on Tuesday.
The 40-year-old woman, a female security guard, and a 43-year-old man have been arrested on suspicion of possessing explosives.
Scotland Yard said the latest arrest was part of an ongoing investigation into where the substances had come from.
The 61-year-old was detained by police on Friday and taken to an east London police station for questioning.
Police said they searched the woman's car after receiving information. Later that night, the same police unit stopped and searched another car on the M11 but nothing was found in the car and the driver was not arrested.
Searches were carried out at residential addresses in Kent and London and no other substances were found.
The 40-year-old woman and 43-year-old man have been bailed until late April.
Scotland Yard has said that the incident is not thought to be terrorism-related.
