They say it can take weeks to develop, rather than just hours or days after an accident.

Researchers found that when brain cells commit suicide in response to damage, the process goes on for weeks.

"A brain-injured patient may look stable, but cells are still dying," Tracy McIntosh, director of the Head Injury Research Center at the University of Pennsylvania.

"Realising this will be important for coming up with ways to recover, regenerate and stem the loss of brain tissue," he said.

One person in the US suffers a traumatic brain injury every 15 seconds, according to the Brain Injury Association.

It estimates brain damage claims more than 56,000 American lives annually.

It says about 373,000 Americans are hospitalised every year as a result of brain injury from accidents such as car crashes and falls.

Unexpected cell death

Dr McIntosh's team damaged the brains of laboratory rats and killed them at regular intervals, looking at the brains.

Early on, cells died the quickest near the surface of the brain where the injury occurred. Cells deeper in the brain started dying later.

After two months, brain cells in the thalamus, the area of the brain responsible for motor function, were dying.

The brain cells were dying in a natural process known as apoptosis, or programmed cell death.

This is supposed to happen in response to injury, but the study found that even undamaged cells were somehow getting their apoptosis genes turned on.

The scientists hope their findings will lead to new treatments for brain injury, perhaps ways to stop the damage before it starts.

The research is published in the Journal of Neuroscience.