Series-leading Ken Roczen won the main event start to finish, and series runner-up Cooper Webb was second.
Tomac was seventh.
He appeared to be on a relatively safe and comfortable trip to the finish line.
He was fifth after Lap 1 at Lucas Oil Stadium, a half-second behind Adam Cianciarulo. Roczen ran in front after taking the lead from Barcia. Webb was in third.
Tomac went by Cianciarulo in Lap 5, and in Lap 7 ran his fastest lap of the race to draw within a half-second of Webb and within 5 seconds of the lead.
In Lap 15, as the four front-runners raced about a second apart, Cooper began attacking the whoops and gained on Barcia before passing him in Lap 19.
Tomac then challenged Barcia for third.
But with three laps to go, Barcia rear-ended Vince Friese at a jump in Lap 24, and Tomac, with nowhere to turn, collided with Barcia’s bike.
Marvin Musquin, Malcolm Stewart and Cianciarulo went by unscathed to finish in the top five.
Tomac remounted his bike in sixth place and finished seventh. Barcia was 19th, and Friese was 14th.
Defending champion Tomac, who entered the season’s third race at Lucas Oil Stadium just one point behind Webb, now trails Webb by eight points and Roczen by 24 points.
Kawasaki teammate Cianciarulo is fourth.
Earlier Saturday, Tomac was second after two timed qualifying rounds, with a best lap of 46.833 seconds. Webb was first at 46.752 seconds. Cianciarulo, Stewart and Barcia rounded out the top five.
Tomac finished second to Roczen in his qualifying heat. In the race, the top five finished in the order they ended Lap 1. Tomac was second the whole way behind Roczen, who led all nine laps, and Zach Osborne, Stewart and Friese rounded out the top five.
Barcia won the second heat, and was followed across the finish line by Webb, Jason Anderson, Dylan Ferrandis and Cianciarulo.
Musquin took the lead in the first lap, but crashed in the whoops in Lap 5 and dropped to seventh. Barcia, Webb and Anderson inherited the top three spots, and Ferrandis and Cianciarulo went by Justin Bogle to finish in the top five.
Tuesday: Tomac displays passing skillsOn Tuesday, Tomac finished third in Indianapolis 2, behind Roczen and Barcia. Webb and Aaron Plessinger rounded out the top five.
Tomac finished Lap 1 in ninth place, but made several quick passes to move into sixth place in Lap 2 and into fifth in Lap 3.
In Lap 8, Tomac passed Plessinger to take over fourth place, and sealed his place on the podium by passing teammate Cianciarulo in Lap 11.
Roczen won the race after leading all 26 laps, and holeshot winner Barcia was second, almost 3 seconds ahead of Tomac. Webb and Plessinger rounded out the top five.
During qualifying rounds Tuesday, Tomac was the No. 2 racer, with a time of 46.316 seconds, behind Roczen, at 46.186 seconds. Kawasaki teammate Cianciarulo was third, at 46.508 seconds, and former Kawasaki teammate Joey Savatgy, now racing for KTM, was fourth, at 46.777 seconds.
Tomac was fourth in his qualifying heat race after starting in second place. Musquin won the heat and was followed by Barcia. Justin Brayton was third.
Roczen won Heat 2, taking the lead after Webb crashed in the fourth of eight laps. Cianciarulo was second and was followed by Stewart and Ferrandis.
Up nextThe AMA Supercross series next goes to Orlando, Florida, for two races, on Feb. 13 and Feb. 20, and then to Daytona Beach on March 6 for the ninth of 15 rounds.