Bruce Jenner told police he is undergoing hormone treatment after being involved in a fatal car accident.

The 65-year-old reality TV star was involved in a four-car pile-up - which left one person dead and seven others injured - and voluntarily agreed to undergo a blood test at a local hospital as part of the investigation into the cause of the crash.

Bruce, who is believed to be transitioning to a woman, was accompanied to hospital by Los Angeles County Sherrif's deputies, and asked if he was taking any medication as he gave a blood sample.

A source told ''[He said that] he was taking very low dosage of hormones that his doctor had prescribed.

''Bruce has nothing to hide, and was very co-operative with cops, and appreciated their professionalism.''

The insider claimed the 'Keeping Up with the Kardashians' star is taking the hormones to grow breasts ahead of planned surgery.

Bruce is said to have taken the accident - which resulted in the death of his neighbour Kim Howe - ''very, very hard'' and is receiving support from his family.

The source added: ''During the entire process Bruce was obviously very, very shocked and inquiring about the other victims -- this has been a total nightmare for him, and Bruce is taking it very, very hard.

''His kids are hunkering down with their dad -- they are a very close-knit family, and will do whatever is needed to help him at this very sad and tragic time.''

It was previously claimed police were looking into the possibility that Bruce was texting while driving but phone records have cleared him and law enforcement are now said to believe the Prius which was at the head of the line of cars made a sudden stop, causing a Lexus to crash into it before the reality star then hit the second car, shunting it into oncoming traffic and hit by a Hummer which killed the driver.

The Prius driver initially told police she stopped at a red light, despite being 300 feet from it, causing officers to believe she was making an illegal U-turn.

Bruce - who has hired respected criminal lawyer Blair Berk - could be cited for following a car too closely, a traffic violation which can also trigger the charge misdemeanor vehicular manslaughter because someone died in the accident.