Russian Grand Prix organisers have launched an official investigation into how a cleaning vehicle left the Sochi track covered in diesel ahead of the first Formula 1 practice session.
Friday morning's track running was delayed for 30 minutes while marshals and fire crews dealt with a diesel spillage that had happened over the second half of the track.
Circuit consultant Richard Cregan told Motorsport.com: "It was a vehicle that was cleaning the track this morning – but was also dropping diesel at the same time.
"It was quite an unfortunate situation really, as it meant the teams had less running time.
"There was quite a lot of spillage, so the track workers did a good job in getting everything ready. But we have apologised for what happened."
Race promoter Sergey Vorobyev said the organising committee would look in to the details of what happened, although welcomed the fact that the timetable had been not impacted.
"All the practice sessions are running according to schedule, so the incident is not critical," he said.
"There was a diesel spill and it took us some time to clean it - which caused the delay.
"So there will be an investigation and it is an internal matter now."