Three-time Indianapolis 500 winner Dario Franchitti fractured two vertebrae and broke his right ankle when his car went airborne into a fence Sunday on the last lap of the Grand Prix of Houston. The accident showered debris into the grandstand, injuring 13 fans and an IndyCar Series official.

Franchitti, who also sustained a concussion, was transported by ambulance to a hospital. The four-time series champion had surgery on his ankle was being held overnight, and IndyCar said a series official was treated for minor injuries.

Houston Fire Department spokesman Ruy Lozano said 13 fans were injured, and that 11 were treated on site at Reliant Park. Lozano said two were taken to the hospital for treatment.

The accident in Turn 5 was reminiscent of Dan Wheldon's fatal 2011 crash at Las Vegas in that competitors had to drive through the wreckage.

It was a sobering moment for race winner Will Power, who broke his back in Las Vegas crash, and for Scott Dixon, who took control of the IndyCar championship race Sunday but passed by teammate Franchitti's car and waved in an attempt to get an update on his condition.

"The smells and the visuals, for me, and even talking to Will, you have the remnants of Vegas popping into your head with you coming around the corner and you can't drive through it because there's a field of debris," Dixon said. "There was no near the amount of damage that we saw (in 2011), but seeing the replay was a big shock."

The accident occurred after contact between Franchitti and Takuma Sato sent Franchitti's car launching over Sato's and into the fence. Parts and pieces from both cars flew into the grandstand and Franchitti's badly damaged car bounced back onto the track. E.J. Viso then hit Sato's car.

The caution came out to immediately freeze the field, preventing Dixon from making a final attempt on passing Power for the win. Dixon won Saturday's first race of the doubleheader weekend and settled for second after Franchitti's crash.

Power initially seemed shaken when he climbed from his car and admitted the accident reminded him of Las Vegas, where he and Wheldon both sailed into the fence.

"I just saw Dario's car and him sitting in it with a lot of damage, and yes, that's what it reminded me of," Power said. "I hate seeing that. We try to keep these cars on the ground."

The accident ended a weekend that saw Dixon move into the points lead following mechanical failures for Helio Castroneves on consecutive days.

Castroneves came to Houston with a 49-point lead over Dixon. But a gearbox problem Saturday when Dixon won allowed Dixon to pull within eight points. Then his gearbox broke on Sunday, and Dixon now has a 25-point lead in the standings and needs only to finish fifth or better in the Oct. 19 finale in California to win his third IndyCar title.

"It's still going to come down to the wire," he said. "It's still going to be the last lap, last corner kind of situation. At least I hope that it ends that way."

Castroneves finished 23rd. He had started on the pole, got a great jump on the standing start to get past Dixon for the lead, but Dixon was screaming on his radio within minutes that Castroneves' car was leaking oil everywhere and it was splashing onto Dixon's tires and visor.

Castroneves said little to his Penske Racing team, but detected a vibration in his car with every shift of the gears after just a handful of laps. The problem worsened and he came to a complete stop on the course at Reliant Park after just 11 laps.

"It's frustrating and disappointing," Castroneves said. "It hurts. It really hurts."

His car was towed to the garage with a broken gearbox, Castroneves made the long walk back still wearing his helmet, and team owner Roger Penske retreated without comment into a team transporter.

The team eventually replaced the gearbox and Castroneves returned to the track, 36 laps down and needing a miracle in Fontana, where the three-time Indianapolis 500 winner was fastest during an open test last week.

"The racing isn't over, we know that. We've seen this thing go up and down," team owner Penske said. "I think Will's performance today shows the speed the team has in the cars. We've been here before, and we'll go to a track we like and maybe it will bring us some luck."

But Penske said Castroneves will likely have to change his engine before the finale which will incur a 10-spot penalty on the starting grid at Fontana.

"It's a long race and we're going to go for it," said Penske, adding "it doesn't make sense, as far as I'm concerned" that reliability and durability failed twice in two days in a season in which Castroneves had been the only driver to complete every lap heading into Houston.

Dixon said his strategy didn't change when Castroneves pulled over Sunday with his problem. Dixon said he saw Castroneves drive over a bump in Turn 1 under yellow that had caused problems all weekend and it caused some of the underwing to fall off of his car. Dixon said oil began pouring out of Castroneves' car.

"He hit really hard going through Turn 1 kink, instead of sticking to the left there, for some reason he went right in the middle and that's when some of the underwing fell off when he hit that hard," Dixon said. "With the amount of oil that was coming out of that thing, I knew it was pretty terminal.

"But at that point you are thinking you still want to win the race, and Will was being super aggressive and trying to win the race and help his teammate."

Penske praised Power for holding off Dixon for the victory.

"With Will out there battling and taking some points away, he got his job done," Penske said.

Power said his job in Fontana will be to help his teammate.

"Going into Fontana, it's all about Helio winning a championship, so whatever I can do," Power said. "If Helio needs to win the race and I need to be somewhere in between him and Dixon, then that's what we're aiming for."