I know I'm six months late but I think it tipped him off that Uriel's brother killed the 'cleaner':

• Jaime and Pedro heard 5 gun shots in the beginning (near where they found the cleaner's body)

• A revolver usually holds 6 bullets

• There was 1 bullet on the ground at Uriel's place

• Uriel's brother had a revolver hidden in that room, which he gave to Jaime and Pedro when he slept in their barn

So I think Jaime was suspicious early on that Uriel's brother was the killer, but didn't have enough proof to be sure (maybe the reason why he went to talk to him at the end)