Relatives of Liverpool fans killed in the Hillsborough disaster have called on the Metropolitan Police to answer claims officers spied on them.
The call comes after Private Eye magazine had a freedom of information (FOI) request turned down by the Met.
The Met has yet to comment, but reportedly turned down the FOI request to "safeguard national security".
Sheila Coleman, of the Hillsborough Justice Campaign, called for police to show "transparency" and admit spying.
Private Eye asked the Met for files it held on the Hillsborough Justice Campaign and the Hillsborough Family Support Group following claims Special Branch officers were involved in surveillance of the organisations.
It reported that police would neither confirm nor deny that it held any papers on the disaster.
Both the justice campaign and the support group have been at the forefront of attempts to discover what happened at the 1989 FA Cup semi-final which saw the deaths of 96 Liverpool fans.
Fresh inquests into their deaths will begin in Warrington in March after the original accidental death inquest verdicts were quashed in 2012 following an independent report.
Louise Brookes lost her brother Andrew Mark Brookes in the disaster and she alleges she has had her post intercepted or tampered with.
She said: "Everything to do with Hillsborough would arrive opened."
Meanwhile, Ms Coleman claimed campaigners' phones were "definitely tapped".
She said: "I represented six families in 1993 and we were aware we were under surveillance.
"A lot of attempts were made to frighten us but it just went with the territory.
"It was appalling. Not so much for me but those who had been bereaved were effectively criminalised.
"I'm disappointed that the government acknowledged a state cover-up but the Met are still holding back."
She said families and campaigners for Hillsborough have been "promised transparency and accountability and the Met should fall into line with that".
Ninety-five victims were crushed to death in Britain's worst sporting disaster, on 15 April 1989, at Sheffield Wednesday's Hillsborough stadium during Liverpool's FA Cup semi-final against Nottingham Forest.
The 96th victim died in 1993 when the Law Lords ruled that doctors could stop tube-feeding and hydration.
As well as fresh inquests, there are two ongoing investigations into the disaster.
The Independent Police Complaints Commission (IPCC) is examining police actions at and after the tragedy, while former Durham Chief Constable Jon Stoddart is looking at the causes of the tragedy and the deaths and examining the actions of a range of organisations and bodies, including South Yorkshire Police.
The IPCC said it was not actively pursuing allegations of spying on families and campaigners.
But a spokesman said: "We are reviewing all material in relation to Hillsborough and clearly if we came across any evidence that this had happened we would investigate it whether or not we have received a complaint."