London police are questioning a 61-year-old man they arrested at the city's Olympic Park on suspicion of supplying explosives.
The man was arrested late Friday and is the third person detained after officers from the Metropolitan Police's Olympic Site Support Unit received a tip and searched a car in a parking lot near the stadium on Thursday.
A female security guard was arrested on suspicion of possessing a small amount of explosives and a man was arrested in Wales as part of the same investigation.
Those two suspects have been released on bail until April and police have said they do not think the incident is terror-related.
Construction work on the flagship stadium for the 2012 Olympics was completed on Tuesday.