Five workers were killed at a recycling site in Birmingham when a retaining wall holding scrap metal collapsed.
Police were called to Hawkeswood Metal on Aston Church Road in Nechells shortly before 09:00 BST.
A spokesman for West Midlands Police said emergency services attended the site but the men were pronounced dead.
Det Supt Mark Payne described it as "a very traumatic incident". It is understood a sixth man managed to scramble free.
He was taken to Heartlands Hospital with a broken leg.
A West Midlands Ambulance Service spokesman said: "On arrival, ambulance crews found that a wall had collapsed.
"Sadly, there was nothing that could be done to save the five men who had been working next to it."
The Press Association is reporting the men killed are believed to be Spanish nationals, originally from Gambia.
They were working next to a "large concrete structure containing metal" when it collapsed, said the fire service.
A specialist team of firefighters is working to recover the men's bodies using heavy lifting gear and cutting equipment.
A spokesman for the Hawkeswood Metal confirmed the deaths were caused by "the collapse of a partitioning wall within the company premises".
"At the present time it is not known why the wall collapsed. The wall had been in place for more than two years and had not been subject to any damage or was not subject to any incident that would have resulted in its collapse," he added.
The company, which is fully co-operating with police, fire crews and the Health and Safety Executive, said it wished "to offer their condolences to the families of all those involved in the incident at this very sad time."
Assistant chief fire officer Gary Taylor said it was a "devastating incident for everyone involved".
BBC News reporter Andrew Dawkins at the scene
Dozens of friends and relatives of the victims have been anxiously waiting outside the plant during the recovery operation.
One of them, Lang Dampha, says two of the dead were his lodgers - and that he had known one of them since childhood.
"I feel sad about it. We come from the same town, Noo-Kunda in Gambia," he said.
He said one of the men "expected to spend the rest of his life here" and his family were coming to the UK on Sunday after he bought tickets for them.
Another man in the crowd, Dantra Sillah, said his cousin was among the dead.
He said the five men who died were from different families, but all knew each other well through attending mosques in the city.
"It is very sad," he said. "We don't have any clear information."
Another onlooker, Manka Sawo, said: "It is a tight community. We pray in the same mosques… they are very hard-working.
"This will greatly affect the community. Yesterday we went to congregation with one of the men for Eid."
Mr Taylor said: "Once the police have finished their initial examination of the scene, our absolute priority will be to ensure that the bodies of those who have lost their lives are recovered in the most timely and safe way possible, and with the utmost dignity and respect.
"The scene is an extremely challenging one, involving significant tonnage of concrete and metal and a structure that is still unstable."
The Health and Safety Executive has been informed and it is understood officers are at the scene.
Det Supt Payne said: "We are still in the very early stages of this investigation to determine exactly what happened this morning.
"Sadly five people have lost their lives and we have several teams of specialist officers supporting their families at this time.
"This has been a very traumatic incident for those who were at the site this morning and we will continue to work alongside the HSE and the other emergency services at the scene."
At a news conference from the scene, he said the victims had been working to recover scrap metal when the accident happened.
He described the collapsed structure as 12 to 15ft high and made of "concrete blocks" that were supporting the scrap metal.
"It appears the wall collapsed on the men and the scrap metal has fallen on top," he said. "It's a difficult and complex scene."
The officer said the recovery operation was expected to last most of the day, with witness statements and CCTV of the scene being gathered.
Police liaison officers are with the families of some of the men helping them to cope with what he described as a "tragic incident".
John Woodhall, from West Midlands Ambulance Service hazardous response team, described workers at the scene being "very upset".
"We've had to bring in heavy lifting equipment, there are a number of hazards we've got to resolve," he added.
Slav Angiyu, a truck driver, was on site when the accident happened.
He said: "The guys came over and said something had happened and we had to abandon the site. My heart goes out to the families of the victims."
Prime Minister David Cameron tweeted: "Shocked to hear about the tragedy in Birmingham. My thoughts are with the families of those involved."
The recycling plant was the scene of a major fire in February this year, when 700 tonnes of scrap metal went up in flames.
In 2012, the company was fined £50,000 after a worker's arm became trapped in machinery.
Hawkeswood Metal Recycling Ltd pleaded guilty at Birmingham Crown Court to a breach of health and safety law.
According to its company website, the firm processes more than 500,000 tonnes of scrap metal each year.
The company, which employs about 50 people, began trading more than 40 years ago and has a customer base that includes local authorities, major PLCs and smaller independent companies.