Several explosions have taken place at a chemical plant in Billingham, Teesside, police have said.
Residents were advised to keep windows closed
Cleveland Fire and Rescue declared a "major emergency" at the site but a fire was later brought under control.
A spokesman said a release of ammonia had been contained but could not say whether a leak was responsible for the blasts which began about 0015 BST.
Plant owner Terra Nitrogen UK said two workers were treated for shock but the public were never in any danger.
The men received first aid treatment for cuts and bruises, added the firm, which acquired the ammonia-production business from ICI in 1998.
Restrictions in place for about two hours after the blasts saw roads around the 400 acre site off Haverton Hill Road closed to traffic.
Residents were also advised to keep doors and windows closed as a precaution during the period.
Police said an investigation will be carried out by the Health and Safety Executive and the Environment Agency.
Terra Nitrogen said an explosion led to a fire "involving mixed gasses" but the cause of the incident was unknown.
"The plant has now been safely shut down and an investigation is under way," it said in a statement.
"The company will co-operate with any separate investigations carried out by the regulatory authorities.
"Terra Nitrogen UK would like to apologise to anyone who has been inconvenienced by the need for the precautionary actions and would thank people for their patience and understanding during the incident."
Peter Honeyman, 28, from Middlesbrough, was driving home with four friends after a night out in Hartlepool.
He was level with the plant at the time of the first explosion.
"The first blast almost knocked my vehicle off the road. I pulled up and heard two more explosions," he said.
"The sky lit up with orange and red flames. I could feel a burst of heat and an awful smell filled my throat."
Les Gunn, who lives about four miles away from the plant, told BBC News 24 he was woken by an "absolutely deafening" sound.
"The second one was absolutely huge," he added. "There was a huge ball of flame."
Joanne Twaits from Billingham said she lives opposite the site. "There was a huge gush of fire and an astonishing thud," she told the BBC News website.
"It woke my two young children and shook the house as I looked out of my window."