Alexander Rossi, a 24-year-old rookie from California, won the 100th running of the Indianapolis 500 after the other leaders were forced to pit for fuel in the closing laps.
Rossi, too, was running out of fuel but he had enough to slowly cross the finish line at Indianapolis Motor Speedway.
Carlos Munoz finished second, Josef Newgarden was third, Tony Kanaan took fourth and Charlie Kimball, another Californian, came in fifth.
Rossi became the first rookie to win the race since Helio Castroneves in 2001.
"I have no idea how we pulled that off," Rossi, a native of Nevada City, Calif., said after he took the traditional gulp of milk in Victory Lane. "I just can't believe that we've done this. I'm just so thankful."
Rossi inherited the lead after Kanaan, Newgarden and then Munoz had to pit for a splash of fuel to complete the race's 200 laps.
"I really think we had a shot," Newgarden said. "It just didn't fall our way."
12:05 p.m.: With less than 40 laps to go, Takuma Sato scraped the wall and brought out another yellow flag at the Indianapolis 500.
Moments before, James Hinchcliffe and Tony Kanaan were swapping the lead as all the front-runners remained tightly packed.
And J.R. Hildebrand was running third. The Californian lost the Indy 500 in heartbreaking fashion in 2011 when he hit the wall with one corner to go in the race.
11:25 a.m.: The two drivers leading the Indianapolis 500, Townsend Bell and Ryan-Hunter Reay, suffered a major blow when their cars collided on pit road.
Bell was trying to pass Hunter-Reay on the outside when he lost control and hit Hunter-Reay's car, sending both cars into the pit wall. The drivers are teammates on the Andretti Autosport team.
The wreck was a boost for James Hinchcliffe and Helio Castroneves, who were running third and fourth, respectively.
Bell and Hunter-Reay dropped to 25th and 26th, respectively.
The field had pitted after a crash involving Mikhail Aleshin and Conor Daly.
Aleshin lost control and hit the wall between the first and second turns on lap 114 of the 200-lap race and Daly's car was collected as well. Neither driver appeared injured.
10:55 a.m.: Sage Karam slammed into the Turn 2 wall to bring out another caution flag near the halfway point of the Indianapolis 500.
Karam was running sixth when the crash happened. The Pennsylvania native climbed from the cockpit and walked away after the accident.
Meanwhile, Helio Castroneves – seeking to become only the fourth driver in history to win the Indy 500 four times – was swapping the lead with pole-sitter James Hinchcliffe.
Ryan Hunter-Reay was third and Josef Newgarden fourth as all the front-runners remained tightly packed at Indianapolis Motor Speedway.
Graham Rahal had moved up to 12th after starting 26th.
10:25 a.m.: Juan Pablo Montoya, the defending winner of the Indianapolis 500, spun and crashed on lap 64.
The Penske Racing driver was running mid-pack when he lost control, slapped the outside wall and came to rest in the infield, bringing out the second caution flag of the race.
Ryan Hunter-Reay was leading, followed by James Hinchcliffe and Townsend Bell.
The Indianapolis 500 is Bell's only IndyCar race of the year, but the veteran driver from California was running a strong race after qualifying fourth in a car prepared by the Andretti Autosport team.
10 a.m.: Ryan Hunter-Reay led the Indianapolis 500 after 40 laps while Tony Kanaan made a charge. The race has been caution-free so far.
Hunter-Reay, who won the race in 2014, held a 1.5-second lead over Townsend Bell. Josef Newgarden was third.
Kanaan, the 2013 winner of the 500, climbed to seventh place after starting 18th.
The field also completed its first round of pit stops. Pole-sitter James Hinchcliffe, who was challenging for the lead, had a poor pit stop and fell back to sixth.
9:30 a.m.: The 100th running of the Indianapolis 500 is underway.
The 33-car field started the race cleanly and Ryan Hunter-Reay, a former winner, quickly took the lead.
Pole-sitter James Hinchcliffe was second, followed by Townsend Bell.
A crowd expected to top 300,000 is pouring into Indianapolis Motor Speedway today for the 100th running of the legendary Indianapolis 500.
It's a mostly sunny morning with temperatures in the low 80s, and the green flag is set to wave shortly after noon local time, or 9 a.m. PDT. The 33 drivers then race at speeds topping 220 mph.
Located in a residential neighborhood west of downtown Indianapolis, the speedway was here before Fenway Park and Wrigley Field were built. The track has hosted the 500 every year since 1911 except for six years during the two world wars.
This year's centennial race is a sellout. The 2.5-mile, rectangular track has about 250,000 grandstand seats and can accommodate an additional 70,000 spectators in its infield.
The track said it issued credentials for about 2,000 members of the media.
Tradition is everything at Indy, such as the pre-race festivities that include singing "Back Home again in Indiana" and the release of thousands of balloons as drivers get the command to start their engines.
And, as always, the winner will take a gulp of milk in Victory Lane.
James Hinchcliffe, a 29-year-old Canadian who nearly died in a practice crash a year ago, will lead the field in the pole position.
Two Americans, Josef Newgarden and former Indy 500 winner Ryan Hunter-Reay, will start alongside Hinchcliffe on the front row.