Natasha McKenna weighed only 130 pounds, but six deputies struggled to restrain her at the Fairfax County jail in February. They handcuffed, shackled, hit and wrestled the mentally ill woman, all with great effort, they said, for more than 20 minutes, before shocking her into submission with a Taser. Minutes later, she stopped breathing. Days later, she died.

But when the Virginia medical examiner’s office issued the cause of her death last week, it declared that the Taser and use of force were not the primary factors. Instead, it was a rare and controversial syndrome that has been cited in dozens of deaths across the country after struggles with law enforcement: excited delirium. Police, medical examiners and some doctors say the condition is real and frightening. Influenced by mental illness or the use of such stimulants as cocaine and methamphetamine, those in its grip often have extraordinary strength, are imperviousness to pain and act wildly or violently. Then, suddenly, some die.

Other medical experts and civil libertarians have questioned the existence of excited delirium and its frequent citation in cases that involve violent encounters between police and members of the public. Some say it is a cover for the use of excessive force by law enforcement.