MANILA — Kian Loyd delos Santos, 17, is just one of thousands of Filipinos shot and killed by the police since President Rodrigo Duterte began a sweeping crackdown on drugs last year.

But the youth’s death last week in Caloocan City, outside Manila, has had an effect that no other police killing has: The Senate, though dominated by allies of the president, has opened an investigation.

Mr. Duterte, known for his brazen promises of impunity for police officers who kill people suspected of using or selling drugs, has personally ordered that those responsible for Mr. delos Santos’s death be taken into custody.

The developments have critics of Mr. Duterte’s drug war cautiously optimistic that the Philippine public, which has been broadly supportive of the crackdown, is starting to see it differently.