I don't think there's a contradiction between "they all did it" and "so far, only many of the big banks are being investigated".
We don't have proof that "they all did it" so perhaps I overstated the case a little. We do know that the UK government contacted Barclays because their rates were higher than everyone else's. Perhaps some of those lower rates were legitimate. Perhaps not.
But if we're going to be pedantically accurate, perhaps you should say "if you have a LIBOR-linked mortgage with Barclays, Citigroup, RBS, UBS AG, ICAP, Lloyds or Deutsche Bank" ....
They all did it. Barclays were the last bank to start doing it, and the first to confess.