The families of victims of serial killer Stephen Port have spoken of their frustration at an inquiry into police failings in the case, calling its slow pace "insulting and distressing".
Port, 42, was handed a whole life term in November for murdering four young men he met through gay dating websites.
The Metropolitan Police admitted "missed opportunities" to catch the killer sooner and at least 17 officers are being investigated by the Independent Police Complaints Commission (IPCC).
The IPCC says it has compiled 7,000 pages of material on the case and that its investigation is progressing.
But relatives of Port's victims are dismayed none of the police officers being investigated have been interviewed.
Mandy Pearson, stepmother of victim Daniel Whitworth, said: "We continue to seek answers and accountability from the police about how, for a whole year, they let us believe that Daniel had committed suicide, in which time Port went on to kill again.
"We really did hope that, with Port now behind bars, the police would be held to account for their actions.
"The fact that after all this time we're still no further forward is insulting and distressing for all of the families.
"However, the MPS should know that none of us will ever give up their search for the truth and we will keep the pressure on."
Mr Whitworth, 21, was killed by Port after being given a fatal dose of the date rape drug GHB.
He and Anthony Walgate, 23, Gabriel Kovari, 22, and Jack Taylor, 25, were all killed between June 2014 and September 2015, their bodies dumped near Port's flat in Barking, east London.
The Met was widely criticised for missing numerous chances to catch the killer, and promised to re-examine another 58 GHB-related deaths over four years in London to rule out foul play.
Lawyers for the families said they were told a first draft of the IPCC report would be complete by this autumn, but that when they met with IPCC commissioner Cindy Butts in March, the time frame was put back to the end of the year, with the final report expected in early 2018.
They also said they were told the number of officers being investigated by the IPCC has risen to 19, though the organisation says 17 officers have been served with notices.
Solicitor Neil Hudgell said: "The families are growing increasingly frustrated by the lack of progress being made by the IPCC.
"We were originally told that the officers would be interviewed in January 2017, but the IPCC hasn't even managed to get dates in the diary and our requests for substantive updates fall upon deaf ears.
"It feels like the IPCC is becoming impotent in the face of stalling by the MPS.
"The families feel that history is repeating itself as the MPS continues to ignore them.
"They are losing confidence that the IPCC has the ability to get to the truth; the longer this drags on, the greater the chance of evidence being lost or forgotten."
The IPCC said since Port's conviction it has carried out a "rigorous process of pre-interview disclosure" on information from the original investigations, the trial and witnesses.
Ms Butts said: "The officers have asked for time to understand and absorb the information that will be put to them in interview, which we have agreed so that the evidence we collect from them is as robust and comprehensive as possible.
"The integrity of our investigation depends on getting this process right.
"While we would have preferred to have been more advanced in our interviews with officers by this time, the investigation is ongoing and we are continuing to progress other investigative steps."
The families of Port's victims are regularly updated on the case's progress, she said, adding: "The investigation team is committed to providing them with answers to their questions and concerns, and will do so as soon as it can."