Police are continuing to question seven men held on suspicion of terror offences after firearms and other weapons were found "hidden" in a car.
They are being held on suspicion of the commission, preparation or instigation of acts of terrorism.
A force spokesman said the arrests "followed a routine stop of a vehicle by police on the M1 motorway in South Yorkshire on June 30".
The car was impounded on suspicion of having no insurance but the driver and passenger were not detained by South Yorkshire Police.
The spokesman said: "Firearms, offensive weapons and other material were later found hidden inside, prompting police to take action to trace and arrest the driver, passenger and others suspected of being involved."
Police said three men aged 23, 26 and 27 from Sparkhill, Birmingham, were arrested on Tuesday morning.
Three more suspects, a 22-year-old from Alum Rock, Birmingham, a 24-year-old from the Moseley area of the city, and a 22-year-old from Smethwick, West Midlands, were arrested on Wednesday evening.
A 43-year old man from Kirklees, West Yorkshire, was detained on Thursday.
Sky's crime correspondent Martin Brunt said the car was not searched until three days after it was seized. It was only then that police were alerted to what was inside and began to trace the men they had let go.
Brunt said the arrests were not Olympics-related but were believed to be part of a "major plot".
Detective Chief Superintendent Kenny Bell, head of the West Midlands Counter Terrorism Unit, said: "As soon as the items were discovered in the impounded vehicle, our priority was to protect the public by pursuing and arresting those we believed to be involved."
Police said the contents of the car were being examined by forensics teams and the suspects' homes had been searched.
Detectives have been given extra time to question the three men arrested on Tuesday and now have until next Tuesday morning to charge or release the men, or apply for a further warrant.