One week after the sudden partial collapse of the huge boiler house building of a defunct power plant in the U.K., three workers remained missing in the “highly unstable” debris, according to emergency services officials. 

The Feb. 23 accident, which occurred as the former Didcot A plant in Oxfordshire, Englend, was being prepared for demolition in March, left one worker dead and five injured.

“The recovery of the bodies and site investigation will be a very complex operation," said local assistant police chief, Scott Chilton, who added that "it will be many, many weeks before it is completed."

Structural engineers, who were not identified, are being brought in to assist with recovery at the collapsed building, which officials said was "highly unstable."

Demolition worker Mick Collings, 53, was confirmed dead in the accident, which lnjured five others.

The 300-m-long, 10-floor building had been largely cleared inside as part of the decommissioning process, according to Birmingham, England-based demolition contractor Coleman & Co.

Coleman took over the site in late 2013 and was due to complete demolition by the end of this year, according to the plant’s German owner RWE NPower. Coleman demolished the plant's three concrete cooling towers in 2014.

Didcot A is a dual-fired power plant that began commercial operations in 1970 and closed in March 2013. Operations of the adjacent Didcot B plant are unaffected. The combined cycle gas fired B plant started operating in 1977.