It is "highly unlikely" three people missing after part of a power station collapsed are alive, emergency crews have said.
One person, named locally as Mick Collings, died when a concrete and steel building at the derelict Didcot A site in south Oxfordshire came down at around 4pm on Tuesday while it was being prepared for demolition.
Five others were taken to hospital, although all but one were expected to be discharged by the end of the day.
Dave Etheridge, Oxfordshire Fire and Rescue Service Chief Fire Officer, said the families of the missing were "obviously distraught".
He said: "We have explained to them we have not picked up any signs of life but we are doing everything we can to locate their loved ones.
"It is highly unlikely they are alive."
He said the operation "may take several days, possibly several weeks".
Police have yet to identify the deceased, although members of the Teesriders Motorcycle Club identified him as Mr Collings, also known as Whitby Mick.
Friends described him as "a big lad with a massive heart" and an "inspiration".
A tribute on the club's Facebook page read: "Words can not describe how gutted I am to be writing this.
"The accident that happened yesterday at Didcot power station yesterday.has killed a good friend and dedicated member of Teesriders Mcc. All our thoughts go out to Lynn and all the family at this very sad time RIP Mick."
Daniel Shepherd wrote: "Absolutely gutted, as a biker one of the nicest blokes I've met, cracking guy rip Mick!"
Shaz Morgan added: "Big lad with a massive heart and infectious laugh. Love and deepest thoughts to Lynn and family x will be sorely missed by all who knew him xx"
Danny Parkinson wrote: "I really can't describe just how much of a loss this is to all who had the privilege to have known Mick. I owe him more than he ever knew. Thanks for everything old mate."
Friend Donna Cotterill said he was "a lovely cheeky man with a smile for everyone and a hug for most".
A 100m cordon has been placed around the scene as the rescue operation continues.
Thames Valley Police Assistant Chief Constable Scott Chilton said: "This is a tragic event and we are doing all we can to support those families.
"The priority at this time is the recovery of the persons missing."
Describing the rescue operation to find the missing trio, Mr Etheridge said: "We have tried their construction site radios and have had no response. We see this as significant.
"We are currently using sniffer dogs, listening devices, drones, and are looking at the possibility of possibly deploying remote control probes into the structure to access the dangerous parts of the site.
"This will enable us to cover areas that are too risky for rescue teams to be deployed to."
He said he had given a personal assurance to the families of the missing that rescue teams will do "everything they can" to recover the loved ones.
The building was due to be demolished when it collapsed, emergency services said, while the Health and Safety Executive (HSE) confirmed it was on-site, investigating.
Didcot A opened in 1970 as a coal-fired power station and was later converted so it could also generate power from natural gas.
It ceased generation in March 2013 and hundreds gathered to watch when three of its enormous cooling towers were blown up in July 2014 after dominating the town's skyline for more than four decades.
The incident comes 16 months after a major fire struck a cooling tower at Didcot B in October 2014.