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Defending Sprint Cup Series champion Brad Keselowski was among seven drivers involved in a wreck in the early going of Sunday's rain-delayed Quaker State 400 at Kentucky Speedway.

The crash occurred on lap 48, one lap after a restart. Keselowski, who won this race one year ago, was bumped by Kurt Busch when both drivers were running on the apron of the track in turn 1. Keselowski's No. 2 Penske Racing car shot up the track and slammed into the wall. Greg Biffle, Dave Blaney and Travis Kvapil also suffered damage to their cars when Keselowski collected them.

This 400-mile race had been scheduled for Saturday night, but persistent rain forced postponement until Sunday at noon ET.

"Wrecks happen, and it takes a chain of events to get there," Keselowski said. "I was using the bottom lane for about two or three restarts in a row and kept getting shuffled back. I was just trying to be patient, because it looks like we would be able to get the whole race in without rain. There was no reason to drive like an animal. Apparently, I was the only one who got that memo. It's just one of those deals."

The front end of Biffle's car briefly erupted into flames. There were no injuries during the incident. NASCAR stopped the race for 18 minutes due to track clean-up efforts.

"I knew I needed to get to the bottom of the track, but my car was a little bit loose getting into the corner, and I just had nowhere to go," Biffle said. "I just couldn't get it turned down to miss it. I guess guys are going for the win on that lap 48."

Busch, who did not suffer any damage to his car, apologized over his team radio for the accident.

"I know (Busch) didn't intentionally wreck me," Keselowski said. "It's just one of those things with the chain of events and the way the cars drive. The track's got that really bad bump down there (turn 1 area). We all know that, so there's no reason to go down there, but I still did."

Keselowski entered this race ninth in the point standings. He has yet to win a race this season.