Two men have died after being injured in a fire on a former steelworks site.
Fire crews were called to the old coke ovens at the SSI site in South Bank, in Middlesbrough, at about 14:15 BST on Thursday.
The site was evacuated and a 250m (820ft) cordon set up.
Tees Valley Mayor Ben Houchen said: “We will examine every single detail of the work that was being conducted. We will make sure we do everything to understand exactly what happened.”
“It is impossible to imagine the pain and distress which this news will have brought the loved ones of those involved,” he said.
Cleveland Police declared it a major incident and fire crews have been monitoring the site overnight, although the fire has now been contained.
Det Supt Tariq Ali said: “The families of those two men have been informed and our thoughts remain with them at this very difficult time.”
The Health and Safety Executive has been informed and all work on the site had been suspended.
The South Bank coke ovens were part of the SSI complex, which had offices and a blast furnace in adjacent Redcar.
The steelworks announced its closure in 2015 when its Thai-based owners went into liquidation and 2,200 workers lost their jobs.
In January, a deal was agreed to transfer more than half of the developable land at the former Redcar Steelworks to the South Tees Development Corporation.