In "The Godfather", how did Don Tommasino become wheelchair-bound?

In "The Godfather," Don Tommasino became wheelchair-bound after being shot while helping Vito Corleone kill Don Ciccio. The backstory is shown in "The Godfather Part II," which revisits Vito's return to Sicily.

Vito travels back to Corleone to avenge the murders of his father, brother, and mother, all ordered by Don Ciccio. Tommasino helps Vito gain access to the elderly crime boss by presenting him as a trusted local associate. After Vito kills Don Ciccio, guards attack the fleeing pair, and a shotgun blast severely wounds Tommasino in the legs.

The injury leaves Tommasino dependent on a wheelchair, but it does not end his influence. He succeeds Don Ciccio as the local Mafia leader and later protects Michael Corleone during Michael's exile in Sicily. The film therefore links his disability directly to his loyalty to Vito and the revenge mission.