"A 30-year-old man was arrested on Sunday at an address in Birmingham on suspicion of preparation of terrorist acts. He was detained under the Terrorism Act and currently remains in police custody," the police statement said.
A 58-year-old man who was arrested on March 23 on suspicion of preparation of terrorist acts and detained under the act remains in police custody, the Metropolitan Police said.
A 32-year-old woman was arrested on March 24 on suspicion of preparation of terrorist acts under PACE and has been released on bail until a date in late March.
In total, 12 people have been arrested as part of the investigation. Nine have been released with no further action.