Bereaved families of July 7 2005 bombing victims have been warned they may have been targeted by the News of the World phone hacking.
The scandal surrounding the Sunday tabloid deepened as sources confirmed "several" bereaved families were feared to have been affected.
Graham Foulkes, whose 22-year-old son David was among the 52 people killed, was among those warned by a senior detective that they featured on a list of potential hacking victims.
Journalists were seeking to access messages left on phones as relatives desperately waited for information in the aftermath of the attack, police believe.
Scotland Yard was unable to comment on the line of inquiry.
Clifford Tibber, a solicitor for a number of the families, said: "The news will no doubt be extremely distressing for the families concerned."
Mr Foulkes told the BBC News channel that the thought of someone listening in to his phone calls was "just horrendous". He said he and his wife had already been wondered "just how low could people go" as they read about allegations surrounding Milly Dowler.
He said a senior police officer told him by phone on Tuesday: "You too may well have been involved in this in 2005."
Mr Foulkes added: "I just felt really upset and sad and sickened that some people would go to those extremes given the distress of 52 families at that time."
He said police told him it was "a very small number of cases, maybe a handful" of families who had been contacted. "I'm waiting to hear from police but we're still quite numb that somebody could have been so cold and hard to have listened to our phone conversations at a moment of such intense distress," he added.