British police have made an eighth arrest in Birmingham as part of what officials are describing as a major counter-terrorism operation targeting Islamic extremists.

West Midlands police said a 20-year-old suspect from the Yardley area was detained Thursday night “on suspicion of the commission, preparation or instigation of an act of terrorism.”

Six men arrested earlier in the week continue to be questioned. A woman who had also been detained was released on bail pending further inquiries. The suspects, all British residents, range in age from 20 to 32 years old.

Authorities said that because the plot was in its early stages, they were not yet aware of the targets or what acts of terrorism were being planned.