Brain damage after an accident or trauma develops slowly, sometimes over weeks, researchers said Saturday.

They said brain cells commit suicide in response to damage and that the process goes on for weeks, not just hours or days as was first thought."A brain-injured patient may look stable, but cells are still dying," Tracy McIntosh, a professor of neurosurgery at the University of Pennsylvania Medical Center, said in a statement.

"Realizing this will be important for coming up with ways to recover, regenerate and stem the loss of brain tissue," added McIntosh, whose findings were reported in the Journal of Neuroscience.

Nearly 2 million Americans suffer from brain injury every year in accidents such as car crashes and falls, the Brain Injury Association said. More than 56,000 die from their injuries.

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, but 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.