Investigations are under way after an explosion at a Teesside chemical plant.
The explosion happened on a pipe at the Terra Nitrogen (UK) Limited site, in Billingham, at 0015 BST on Thursday.
Emergency services declared a major emergency after a fire started involving mixed gases including hydrogen, nitrogen and ammonia.
Two people working at the site were treated for minor cuts and shock. Residents reported hearing the blast and seeing a ball of flame.
Roads were sealed off around the scene and residents were advised to keep doors and windows shut as a precaution. The plant was shut down.
By about 0300 BST, police said the incident had been scaled down and there were no off-site implications or risks to the public.
UK manufacturing director of Terra Nitrogen UK Limited Ashraf Malik said: "We 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.
"The company would also thank our employees and the emergency services for their efforts."
A company spokeswoman added that there had been no other incidents at the site since Terra Nitrogen took over from previous owners ICI in 1998.
The site is routinely regulated by the Environment Agency and Health and Safety Executive. It was last visited by Environment Agency inspectors in May and no problems were found.
What the huge Billingham plant looks like from the air
Terra Nitrogen produces nitrogen products used in the chemical industry and agriculture. It employs 400 people in the UK, including 250 people on Teesside.
A spokeswoman for the company said the explosion happened on a pipe at the ammonia plant.
The company said it had started a full investigation. The Health and Safety Executive and Environment Agency will also be investigating.
Supt Ian Richards from Cleveland Police said the emergency services had the situation under control in a relatively short period of time.
He said: "Fortunately what was initially perceived as a major incident was quickly dealt with.
"The emergency services and petrochemical industry on Teesside regularly exercise to ensure such incidents can be dealt with effectively.
"Their expertise and training to deal with such situations and protect the public is nationally recognised."
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."
Here is a small selection of some of the comments we've received about the explosion
I live in Yarm, about 10 miles from Billingham. I was on my computer at the time listening to music. I heard a noise outside, and turned my music off and looked out of my window. There was a bright orange glow on the horizon, and a roaring sound - like an aeroplane taking off, but louder. I immediately thought there must have been an explosion at the chemical plants.
Andrew Duffell, Yarm, Teesside
There was a huge deafening sound and the sky glowed orange, flames flew into the air from Haverton Hill!
Lorren Mitchell, Middlesbrough England
I live in Ingleby Barwick and i heard the explosions very loud from here. At first we thought it was this low flying plain but then we heard it loud very loud.
Lee Simmonds, Ingleby Barwick United Kingdom
I had just got to sleep when my windows were blasted in with glass all over floor, their is an awful smell, I phoned police and waiting help. Their are towers of flames everywhere over plant way.
Joshua Fillford, Teesside
I live around 3 miles from the chemical plant and was awoken around 12:20am by an almighty bang, I jumped out of bed and rushed to the front door, looking down the road I could see flames over 100 feet in the air and a very loud roaring noise.
Wayne Armstrong, Billingham
I was sat in my front room when I heard a huge roar that sounded like a fighter plane taking off. We could here that for at least 10 mins before it appeared to die down. I'm just glad it wasn't bigger.
Kev Jones, Middlesbrough England
We'd just gone to bed when we heard a huge roaring noise outside. My wife jumped out of bed and looked out the window to see the sky orange. We were both astonished at the site of flames high above the houses. The sound of the blast must have lasted almost a minute, slowly fading to nothing.
Peter Spaldin, Billingham
I was woken with a huge whoosh which sounded like a huge flare going off but a lot louder. I had a look outside and saw the neighbours on the street. I live close to the plant and at first thought it was a low flying plane. Because it lasted for so long I was getting quite concerned. Suddenly it stopped and the next thing I could hear was sirens.
Dean Richardson, Low Grange Estate, Billingham, UK
I was walking back home with a friend around 00.16 and a huge bang sent us both to ground and a huge fire ball lit the sky.
Michael Saunders, Thornaby
I'm at Teesside University in Middlesbrough and I was revising for an exam when I heard this big whooshing sound that seemed to last about 2 minutes. It sounded like a tidal wave in the distance! If anything's leaked out into the air then I'm certainly not hanging around for my exam!
Matt Peters, Middlesbrough, England
I heard a roar like Concorde taking off, went out to see what it was, realised it sounded like a burn-off used to when I worked right next door to ICI, and came straight back in again! I live 15 miles away.
Susanne Convery, Stokesley, North Yorks
I was dozing shortly after going to bed and was woken by a MASSIVE explosion. It was so loud the house shook.I live about 3 miles from the incident.
Lee Carberry, Stockton-on-Tees