The waiting lasts about six hours. Finally, the strength of the tide begins to slacken, and the four police divers pull on their scuba gear and prepare to drop again into the dark waters of the Harlem River, to blindly grope its floor for a single object.

The gun.

Specifically, the Glock semiautomatic, .40-caliber handgun used to shoot and kill Officer Randolph Holder on the edge of East Harlem on Tuesday night. Officer Holder had been chasing a gunman who turned and shot him in the head, the police said. A suspect, Tyrone Howard, was arrested and charged with murder.

The suspect was himself shot and wounded by another officer a few blocks north, but was able to get rid of his pistol. A combing of the spit of land between the river and Franklin D. Roosevelt Drive turned up nothing, so the search turned to the water. Teams of police scuba divers are working at all hours, during slack tides, when the waters are calmest, searching for the gun.

Even with a suspect in custody, there are many practical reasons for finding the weapon, an important piece of evidence at trial and most likely a wealthy source of information about its origin and potential history in past crimes.