An army bomb disposal team was sent to a house in northern England after police arrested four men on Tuesday on suspicion of planning acts of terrorism.
Three men, aged 22, 36 and 41, were held after raids at their homes in Sheffield and a 31-year-old was detained at an address in nearby Chesterfield.
“The arrests were intelligence-led and pre-planned as part of an ongoing investigation by Counter Terrorism Policing North East and (domestic security agency) MI5,” West Yorkshire Police said in a statement.
They added: “The public may have heard loud bangs at the time police entered the properties. We would like to reassure them that this was part of the method of entry to gain access.”
Police said the bomb disposal unit had been sent to the Chesterfield house and that nearby residents had been evacuated as a precaution.
There were no details about what the men were suspected of planning. They are being questioned on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
Britain has suffered five militant attacks so far this year, four of which led to loss of life, while security chiefs say another nine have been thwarted.
The country remains on its second-highest threat level meaning an attack is considered highly likely.