The Verizon IndyCar Series has said that it is investigating an incident that took place on Sunday during the Firestone Grand Prix of St Petersburg in which a spectator was hit by flying debris.

According to the Tampa Bay Times, Brigitte Hoffstetter was walking through the concession area behind the grandstands at turn 10 when she was hit on the head by carbon fibre debris, believed to have come from one of the race cars on the track shortly after the race started.

"Could have been very bad," her husband Greg told the newspaper via text message. "It's amazing she is alive."

Mrs Hoffstetter was treated at the scene by St Petersburg Fire and Rescue and then transported to a nearby hospital, where she remained in the trauma centre overnight after being evaluated with a fractured skull.

The race promoters confirmed the incident in a statement released on Tuesday which read: "On Sunday, March 29, a spectator was injured during the Firestone Grand Prix of St Petersburg. She was treated on scene and transported to Bayfront Medical Center for further treatment where she was admitted and remains in stable condition. We wish her well and a speedy recovery."

A statement from IndyCar itself on Tuesday afternoon confirmed that an investigation into the accident was underway, with the series quick to point out that "the safety of IndyCar's fans and participants is of the highest priority."

"The series is investigating an incident where a spectator was injured at St. Petersburg on March 29," the official statement continued. "This process is consistent with IndyCar's procedure following all race incidents and part of its persistent effort to make safety improvements. IndyCar wishes the spectator well and hopes for a speedy recovery."

It has been reported that IndyCar's president of competition Derrick Walker remained in St Petersburg after the weekend to stay close to developments and offer any possible support to the Hoffstetter family.

The Sunday afternoon race saw several full course cautions for debris on the track, many as a result of racing contact between cars that caused parts of the new aerodynamic bodywork kits that were being used for the first time in competition to break off and fly into the air.

"Amazing the amount of stuff that fell off," said Penske driver Will Power after the race, which saw more than half a dozen front wings replaced during the 110-laps, which lasted for two and a quarter hours with five cautions in total, three of them explicitly for debris.

Although in most instances the debris landed back on the track and was retrieved by track workers under the ensuing caution, it appears that at least one large piece of debris managed to clear the catchfence as well as grandstands located 100 yard away from the track to cause Mrs Hoffstetter's injuries.

Mrs Hoffstetter currently remains hospitalised, but is expected to make a full recovery.

The weekend also saw a serious accident for a driver in the build-up to the Pro Mazda Championship Presented by Cooper Tires support race, when JDC Motorsports' Michael Johnson made contact with the wall in turn 3 during opening practice on Friday morning.

Bayfront Medical Center trauma surgeon Dr Jeffery Johnson stated that Johnson was in ICU under observation with fractures to the hip and pelvis. There have been no further medical updates regarding Johnson's condition in the days since his initial admission and treatment.

Johnson is the first paralysed driver to hold an IndyCar race license, competing in a specially-equipped race car with hand controls. A dirt track racing accident ten years ago when he was 12 years old left him with fractures to the T5 and T6 vertebrae in his back, which resulted in permanent paralysis from the mid-chest area down.