A 61-year-old man has been bailed by police after being arrested at London's Olympic Park site on suspicion of supplying an explosive substance.
He was the third suspect taken into custody 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, was questioned and bailed.
On Wednesday, a 43-year-old man was arrested at his home in Wales on suspicion of possessing explosives.
All three were bailed to late April, with Scotland Yard saying the latest arrest was part of an ongoing investigation into where the substances had come from.
Police searched the woman's car after receiving information, they said.
Later that night, the same police unit stopped and searched another car on the M11 but nothing was found in the vehicle and the driver was not arrested.
Searches were carried out at residential addresses in Kent and London but no other substances were found.
Scotland Yard has said the incident is not thought to be terrorism-related.