Demolition of the decommissioned Didcot Power station has been set back at least two years after a collapse that left four dead, the BBC has been told.
It is a year since Ken Cresswell, 57, John Shaw, 61, Michael Collings, 53, and Christopher Huxtable, 34, were killed when the boiler house came down.
Relatives are visiting the site where a memorial is being held.
RWE Npower planned to have it cleared by the end of 2017, but an expert said the collapse had delayed it until 2019.
'Saddest of days'
A minute's silence was held at the site at 14:00 GMT.
Mark Coleman, chairman of Coleman & Company, which employed the four victims, said: "Today is the saddest of days. It is exactly one year since the tragic events at Didcot claimed the lives of Chris, John, Kenny and Mick."
Speaking on behalf of the families, lawyers Irwin Mitchell said: "The families want to know why this terrible event occurred and we will support them in getting the answers they deserve."
Thames Valley Police said the investigation, which the HSE are also involved in, is continuing on-site.
Det Ch Insp Craig Kirby said: "To date more than 1,300 statements have been taken, over 2,600 physical and documentary exhibits have been collected, along with 60,000 still images and video footage."
RWE NPower has not announced a new demolition contractor and said there was currently no timescale for work being completed.
An Npower spokeswoman said "due diligence" and consultations with various agencies would have to be completed again once a new firm was appointed.
Mark Anthony, editor of Demolition News magazine, said at the moment the site was "in limbo".
Debris from part of the building brought down in a controlled explosion in July still remains, and the three cooling towers and a chimney also need to be demolished.
The four deceased demolition workers were employed by Coleman and Company, which withdrew from its contract in September.
The coal-fired facility was closed in March 2013 after 43 years of service, and a major incident was declared on 23 February.
The last of the bodies was not discovered until September and the cause of the collapse is being investigated jointly by police and the Health and Safety Executive.
Mr Cresswell and Mr Shaw, both from Rotherham; Mr Collings, from Teesside; and Mr Huxtable, from Swansea; will be remembered at the site with the laying of flowers and a period of silence.
Mr Anthony said it was "too early to know what went wrong" and any prosecution resulting from the collapse might not be concluded until 2020.
RWE Npower said a new contractor should be in place "within weeks" but there was currently no timescale for work to be completed.