Four men and a woman from Birmingham remain in custody today after being detained in an operation involving MI5 and authorities in Belgium and France.
West Midlands Police said specialist officers were sent into a property linked to the inquiry as a “precautionary measure” while it was searched.
Police officers patrol Gatwick Aiport's south terminal shortly after the Brussels attacks (Getty Images)
“During a search of an address in connection with the counter terrorism arrests in Birmingham a suspicious substance was found,” a spokesperson for the force said.
“There was no immediate danger to the public and it was not necessary to evacuate any homes.”
The force would not give a description of the substance or respond to rumours it could be hazardous or explosive in nature.
Detectives detained three men, aged 26, 40 and 59, and a 29-year-old woman in Birmingham on Thursday night, while a 26-year-old man was arrested at Gatwick Airport in the early hours of Friday.
In pictures: Terror attacks at Brussels airport
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They are being questioned over possible links to the network of Isis militants who carried out the terror attacks in Paris and Brussels.
French and Belgian officials would not confirm any link to their own investigations when contacted by The Independent but West Midlands police said the arrests formed part of an extensive inquiry into “any associated threat” to Britain following terrorist atrocities in the French and Belgian capitals.
Assistant Chief Constable Marcus Beale said a number of properties in Birmingham were searched after the pre-planned and “intelligence-led” arrests.
"This action forms part of an extensive investigation by West Midlands Counter Terrorism Unit, together with the wider counter terrorism network, MI5 and international partners including Belgian and French authorities to address any associated threat to the UK following the attacks in Europe,” he added.
"There was no risk to the public at any time and there is no information to suggest an attack in the UK was being planned."
All five people arrested were held on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
Paris attacks: Abrini arrested
It comes just a week after the arrest of Mohamed Abrini, who admitted being the "man in the hat" pictured alongside two suicide bombers who blew themselves up at Brussels Airport last month.
Some 32 people were killed and nearly 300 were injured in suicide blasts at the airport and the city's Maelbeek Metro station.
Abrini was the last identified suspect still at large following the 13 November attacks in Paris, which left 130 people dead.
He was an alleged accomplice of Salah Abdeslam, who is also in custody after being found hiding in Brussels months after aborting his own suicide bombing mission in the French capital.
