A brother and sister from Birmingham have been arrested under the Terrorism Act, West Midlands Police have said.
The pair were arrested on suspicion of possessing documents likely to be of use to someone committing or preparing an act of terrorism.
The pair, aged 23 and 18, were arrested on Thursday morning at an address in the Small Heath area of the city.
Police said the arrests had been pre-planned and not made in response to any threat to public safety.
Computers and other electronic devices have been seized and will be forensically examined.
Police said the investigation was likely to take some time due to the number of files which will have to be assessed.