And it wasn't circumstantial. Ford was a local who was immediately identified by the family during the act. The surviving family members all identified him, and their story also matched perfectly with that of the accomplice - until he later started creating overtly fabricated tales. Ford was also offered a plea deal that would have spared his life. He instead decided to try his luck with this whole 'I was just in the car, honest' story.
That he felt people would just believe him is what you get from sociopathy + narcissism. He was convicted after less than 7 hours of deliberation. Notably Texas isn't as fast and loose with the death penalty as its reputation either - in those 7 hours they had to determine if he would likely be a continuing threat to society and whether there were any possible mitigating factors that might otherwise yield preference to life imprisonment.
[1] - https://law.justia.com/cases/federal/district-courts/FSupp2/...