A number of controlled explosions have been carried out at a flat where "potentially volatile" substances were found, police said.
A 55-year-old man remains in custody and up to 200 people are still out of their homes since the discovery by police during a search of a flat in Newhall near Swadlincote in Derbyshire yesterday.
Homes within 100 yards of Belvoir Crescent were evacuated and police said they were working with the county council, police and voluntary groups to house the displaced residents.
Superintendent Paul Callum, who is in charge of the incident, said: "The safety of the public is paramount in these situations.
"We apologise for any inconvenience to the evacuees. We will let them return to their homes as soon as we can be sure the area is safe."
Local resident Leslie Wood chose to stay with friends during the evacuation, and said police had told him it would last at least 24 hours.
He told Sky News: " T here's been police activity during the afternoon and the road was cordoned off, and they came to me and said the situation was you've got t o vacate the premises."