What would thier injuries have been?

I know this seems a morbid question but I have to get some closure and in my case I always need details. In 2006 my cousin and his girlfriend were involved in a horrific road crash....They were driving so fast that they went under the back of a lorry trailer. My cousin and girlfriend were killed instantly, I suppose decapitated. In the back were 3 youths without seatbelts who also died instantly. Can anyone tell me why they would have died....what would thier injuries have been. I don't want to appear as if I am some sort of sicko I just need to have this mystery cleared in my mind.......the inquest just spoke of multiple injuries but to me that seems so impersonal. Please only answer me with genuine answers........please don't call me names and judge me........I just need to know what may have happened. Thankyou