It's 'was + to blame'.
Eight people died in an explosion at a government building in Russia's Ingushetia province.An initial investigation suggested a gas leak was to blame
Can someone explain was to ?
It's 'was + to blame'.
It refers to the past, there was an explosion, an investigation decided that a gas leak was to blame; it was caused by a gas leak.
The 'was' and the 'to' have no connection. Think of a synonym for 'to blame', or paraphrase - '... a gas-leak was culpable/the cause'. The word 'culpable' preserves the idea of 'blame', which makes it slightly more appropriate*, as what the investigation wanted was not just the cause - it wanted to find something they could blame.
*But I think 'to blame' is better, as 'culpable' often refers to a person (or to something done/not done by a person = for example 'culpable neglect').
b