British police arrested an 18-year-old man at Manchester airport on Saturday on suspicion of terrorism offenses related to Syria's civil war.
The teenager from the central English city of Birmingham had posed no risk to the public, West Midlands Police said in a statement, without giving further details about why he was held.
The detention was not connected to Friday's arrests of five other people from Birmingham, police said. Those were linked to last month's suicide bombings in Brussels, which killed 32 people, and November's attacks in Paris that left 130 people dead, a security source said.
Earlier, police in Spain arrested a couple believed to be part of a group that supported and recruited Islamic State fighters, including individuals that had carried out suicide bomb attacks in Syria.
Belgian Prime Minister Charles Michel warned earlier this week that further threats to Europe were still a possibility, and that security forces had to stay alert.