Gwendolyn Smalls’s brother, Linwood R. Lambert Jr., died in police custody in May 2013 after being repeatedly stunned by South Boston, Va., police. (Steve Helber/Associated Press)

THE DAY Linwood Lambert Jr. died in South Boston, Va., in May 2013, the police department there issued a curt news release with the headline “Subject Passes Away in Police Custody.” The announcement failed to mention that Mr. Lambert, 46, an African American man, died shortly after three white police officers shot him with Tasers 20 times in the space of about 30 minutes.

Other salient details were missing from the release, including the facts that he was Tasered repeatedly while sprawled at the entrance of a hospital emergency room and again minutes later, while he was shackled in a patrol car’s rear seat.

Video footage of the incident, released recently by a judge’s order and reported in depth by MSNBC, suggests it is a case study in abuse by police ignorant of best practices in the use of stun guns. Their actions violated guidelines, including the local police department’s own policy forbidding Tasering suspects in handcuffs. State police and prosecutors are investigating whether the officers, who have all been promoted since the incident, committed a crime.

Over a seven-year period in the 2000s, 334 people died in the United States after being shot by police using stun guns, according to Amnesty International USA; many more have died since then. Although coroners in such cases often attribute the deaths to other causes — in Mr. Lambert’s case, a medical examiner cited “acute cocaine intoxication” — the use and overuse of stun guns is often at least a contributing cause.

The incident involving Mr. Lambert unfolded when police in South Boston, a small town near the North Carolina border, went to a motel before dawn in response to noise complaints. They found Mr. Lambert there, delusional and acting oddly.

Handcuffed but assured he was not under arrest, Mr. Lambert was taken to a hospital where, upon arriving, he became agitated and kicked out the squad car’s rear window, bolted from the back seat and ran toward the emergency room entrance, crashing into its sliding glass doors. The three officers immediately pulled out their Tasers and began firing, not stopping even as Mr. Lambert writhed and moaned on the ground. One of them, Cpl. Tiffany Bratton, shot him 10 times in the space of two minutes, each shot delivering 50,000 volts. The Tasering continued even after Mr. Lambert, by now shackled at the ankles and reinserted in the squad car, slumped in the back seat.

Rather than seeking the medical attention Mr. Lambert needed, the police took him to jail. They seemed slow to notice when he lost consciousness en route; he was dead by the time he was returned to the hospital.

The officers’ actions flouted federal guidelines for the use of stun guns, including warnings that the jolts not exceed 15 seconds in total — Mr. Lambert was probably shocked for well over a minute; that subjects should not be shot with more than one Taser at a time; and that Tasers should not be used against suspects who are handcuffed or under the influence of drugs (which Mr. Lambert admitted to the police he was). Critically, guidelines stress that anyone shot repeatedly with stun guns get immediate medical attention. Mr. Lambert, Tasered at the hospital doorway, received none before he died.

“Why are you trying to kill me, man?” Mr. Lambert, supine, asked as the police Tasered him. Why, indeed.