Doc Rivers had no clue as he made the right-hand corkscrew turn on the exit from the Prudential Center tunnel.

Rivers left what he thought was the last practice of the Celtics’ regular season. He had driven through the sunshine down the Mass. Pike, past Fenway Park, where earlier his second favorite baseball team had won in the bottom of the ninth inning.

Rivers had only to merge onto the city streets and drive to his residence on Boston Common, then walk the 41⁄2 blocks to the Boston Marathon finish line. The race’s winners had long since crossed, but the victors — the ordinary people sharing their extraordinary accomplishment — would still be coming down Boylston Street in various states of motion.

To wade into the crowds along the sidewalk is a Rite of Spring, a baptism for some, a renewal for others.

Rivers had partaken in the past, and would do so again on this glorious Monday. Or so he thought until his car again found the sunlight. And the darkness.

“I didn’t hear or see anything, but you just saw the people running and the ambulances,” he said, describing the scene near Copley Place. “It was just everything. It was hysteria. It was crazy.”

The bombs had exploded on the last miles of his trip down the Pike, and the scene of crime and tragedy was unfolding before his windshield.

“I didn’t know,” Rivers said. “Honestly, I didn’t know what was going on. You know, you come out (of the tunnel) and all of a sudden you see all this, and you don’t know what’s going on. Then I turn the radio on and find out what had just happened. From there I’m just trying to get home. That’s all I was trying to do. It took forever. It took an hour, I think, to go five blocks.”