Hong Kong police have fired more than 1,800 rounds of tear gas as the mass protests gripping the city have become increasingly fraught.
Experts who reviewed videos for the New York Times said the police had repeatedly violated standards for safely dispersing crowds, putting protesters and bystanders in serious danger.
Hong Kong police have defended their use of tear gas, saying the protests have become more violent in recent weeks, with demonstrators embracing more aggressive tactics. Activists have thrown bricks, blocked streets and set fires.
The police say they have acted with restraint. Often, they hold up large banners warning the crowds about tear gas before they fire. But a Times review of dozens of episodes involving tear gas show police, at times, have used methods that experts describe as indiscriminate and excessive. Many recordings show officers deploying tear gas against crowds that appeared nonviolent and that were not attacking police.