A 15-year-old boy has been arrested after suspected explosive substances were found at a house.
Army bomb disposal experts were called to the terraced council property in Hull along with scores of police, firefighters and paramedics following a tip-off.
A number of families were moved out of their homes as a precaution.
The teenager was arrested in the early hours of this morning along with a 49-year-old woman and a 56-year-old man.
A multi-agency operation, led by Humberside Police, was continuing tonight more than 15 hours after officers arrived in Kilnsea Grove in the east of the city.
Some of those evacuated were let back into their properties after the bomb disposal team left late this afternoon but a handful of homes remained cordoned off this evening.
A number of police and fire service vehicles were still parked outside the house and officers have erected a blue forensics tent on the road in front of the property.
Earlier, dozens of people living in the area gathered behind the police cordon to watch the soldiers go into the property.
One man said: "I can't believe what descended on us. There's army, police, ambulances, four-wheel drive ambulances, fire engines. Everything's here.
"They came in the middle of the night after a tip-off about stuff in the house. Then all hell broke loose. I've not seen anything like it."
Another Kilnsea Grove resident said: "They've just said they've found explosives inside and we're to stay well away.
"They've got everyone out just in case. You can't be too careful."
A Humberside Police spokesman said they received a call at 2.49am today and confirmed they were "investigating a report of substance(s) which are believed to be inside a residential property".
The spokesman said the Army's Explosives Ordnance Disposal Unit had been deployed as part of a multi-agency response.
Detective Inspector Jason Gourley said: "Our officers arrived at an address in east Hull in the early hours of today.
"Since then, we have arrested three people who are currently assisting us with our enquiries in connection with substances believed to be at a property on Kilnsea Grove.
"We are working closely with our partner agencies to ensure the area is safe for the local community, which is why we have put a cordon in place.
"We hope that residents can return to their properties as quickly as possible, but public safety is paramount, so we hope that everyone will obey the police cordons and we will let them know when the police has finished carrying out all relevant enquiries."
The incident is not being treated as terrorist-related, police said.
Later, police said most people in Kilnsea Grove had returned to their houses.
The spokesman said: " Agencies recovered substances from an address on Kilnsea Grove this afternoon, which were taken to a site in Hull to be disposed of safely.
"The substances have been made safe and seized as evidence. No one was injured."
Mr Gourley said: "We appreciate that many people of Kilnsea Grove would have been inconvenienced when they were asked to evacuate their homes, so we would like to thank them for their patience, whilst we conducted inquiries and made the area safe for them to return.
"The evacuation was necessary in order to ensure every local resident was safe during the investigation, which was our top priority.
"I would like to reassure members of the public that we believe this is an isolated incident and we don't believe this incident is connected to any terrorist organisations."