Armed police have raided a block of flats near to Manchester Piccadilly Station.
Video footage showed officers, some clad in balaclavas, piling out of a van on Wednesday afternoon, before storming a building on Granby Row.
The station was briefly closed during the raid, police said.
A witness described: “I saw down there about four armed officers coming out from the back of the building.
“Again, helmets, face masks, machine guns and getting into two unmarked cars and driving away.”
A Greater Manchester Police spokeswoman said: “Officers have this afternoon carried out a search at an address in Manchester City Centre as part of the investigation into the horrific incident at Manchester Arena.
“That search is ongoing.”