Forensic teams, police, fire and ambulance crews are stationed outside a Northampton house as part of a major incident, while witnesses have seen officers leaving a house wearing 'gas masks'.
A property in Tresham Green, Ryehill is cordoned off as part of what appears to be a major operation.
Currently, two fire engines, an ambulance, a major incident support vehicle, two police vans and a police 4x4 vehicle are positioned outside the property.
Witnesses have described how emergency services have gradually arrived at the property throughout the morning. They could not say whether there had been a raid on the home.
An NCA spokesman said: "Officers from the National Crime Agency, with support from Northamptonshire Police, carried out a search at an address in Tresham Green this morning as part of an investigation into the supply of fentanyl. A man aged 32 and a woman aged 31 have been arrested and are currently being questioned. No further information is available at this stage as enquiries are ongoing."
One officer at the scene told the Chron that the response was part of a"multi-agency operation".
A police van belonging to Derbyshire Constabulary is among the vehicles at the scene.
Updates to follow.