Dozens of households have been evacuated for a second time as police and specialist scientists investigate reports of illegal chemicals being stored at a property in Bridlington.
The pre-planned operation is not terrorist-related, officers said.
People living within the police cordon were first told to leave their homes on Thursday as police, scientists and soldiers from the Royal Logistics Corps Explosive Ordnance Disposal (EOD) squad worked in the property, believed to tbe base of a chemical supplies business.
But but they were allowed back as work stopped for the night.
The evacuation began again at 8am on Friday.
A force spokesman told residents returning to their properties overnight: “We would like to reassure you that specialist scientists have advised us that it is safe for you to return to your property. Police officers will remain in the area overnight to keep you and the scene secure.”
He said the second evacuation was necessary due to potential dangers while work was ongoing.
He said: “This is because there is a risk to your safety while they are actively working in the house.
“As a precaution it will be necessary for us to evacuate houses and prevent access again tomorrow. We are striving to allow all residents to get back to normality as soon as possible.”
He added: “We must stress to you that we don’t underestimate the impact of asking people to leave their homes. We would like to reassure you that public safety is our number one concern and it is a necessary decision.
“The response from the public has been really positive and we’d like to thank you for your cooperation and continued support, and apologise for all the inconvenience caused.”
On Thursday residents were told by the police: “As a precautionary safety measure houses in the immediate area were evacuated and at this time officers from the EOD remain in attendance making an assessment of a variety of substances within the property.”
Superintendent Ed Cook said the evacuation was necessary due to the risks involved when the chemicals are moved.
Mr Cook also warned people in the area that there would be a series of “very small” controlled explosions on Friday.
He said: “We were able to allow residents of Oxford and Cambridge Street, Bridlington, to return to their homes temporarily last night as the risk from the chemicals was contained and the area was safe.
“This morning we will be moving the chemicals which introduces an element of risk, and in order to protect the public and ensure their safety we have evacuated them again.
“The chemicals will be disposed of safely with the support of the EOD and forensic scientists. There is likely to be a series of very small controlled explosions in the area.
“Our primary concern is to keep residents safe.
“We do understand that it is inconvenient, and will make every effort to resolve the situation as soon as possible.”
East Riding of Yorkshire Council said it had made The Spa Bridlington complex available for residents who were evacuated and were unable to make alternative accommodation arrangements.
A spokeswoman said that more than 30 people attended The Spa Bridlington on Thursday while the police cordon was in place and were provided with food and drink. She said some people were able to watch a show.
The customer service centre at the town hall in Bridlington will operate as a drop-in centre for information for residents and will remain open until 8pm, she said.