But then he saw two young men running away from the scene, and that's when he took action, going from bystander to hero.
In his living room with his wife and four sons, Morales admits it's an unremarkable ending to an unforgettable day.
"I just saw there's kids laying out in the street, and the bus was turned over, and I just reacted," said Morales.
Morales is the superintendent of a construction site in Boyle Heights. Monday afternoon, at quitting time, his workers told him about the crash at 1st and Soto streets: a school bus full of students was flipped onto its side.
"One of my guys grabbed me and sort of pointed up the street and said, 'Hey, I think those are the two guys that caused the accident.' So before I knew it, I was doing a foot-chase up the street chasing these guys."
Morales saw them trying to hide next to carport. And the former Golden Gloves boxer didn't hesitate.
"I just grabbed them, put them out in front of me and just walked them back to the first policeman I could see," said Morales. "He didn't say anything. And I said, 'I think these are the two guys you want.' So he kind of just looked around for a minute and handcuffed one of them, and he grabbed the other one, and that's when I left. I had to go back and do my paperwork."
Morales's family says they're not surprised that he took action.
"He's a pretty tough guy. I mean, he used to box when he was young, for 10 years, so it's in him, you know?" said Mario Morales Jr.
But Morales says he's no hero.
"I don't think they would have gone too far. They were pretty messed up," said Morales. "I think that's why they hid around the first car they could find."
Morales said he feels very badly for the family of the pedestrian that was killed. He said the only good news is that more people were not seriously hurt.