Detectives have been given more time to quiz six suspects held over what police called "international-related terrorism".
Four men from Derby aged 22, 27, 35 and 36, a 27-year-old man from Burton upon Trent in Staffordshire and a 32-year-old woman from London were arrested on Monday on suspicion of engaging in the preparation of an act of terrorism.
Counter-terrorism officers now have until next Monday to question the suspects after being granted a warrant of further detention for seven days.
Searches took place at properties in Derby, Burton and London and an Army bomb squad was called to the Normanton area of Derby as a "precautionary measure". No homes were evacuated.
The investigation is being led by the North East counter terrorism unit (Nectu) supported by officers from Derbyshire, Staffordshire and the Metropolitan Police.
A Nectu spokesman said: "The arrests are linked to International-related terrorism."