LOS ANGELES — In the midst of a frenzied weeklong manhunt, Mayor Antonio R. Villaraigosa put together one of the largest rewards in Southern California history: $1 million for information leading to the arrest and conviction of Christopher J. Dorner, the former Los Angeles police officer who was wanted in multiple killings last month.

Hundreds of tips poured in over the next two days. And two claims to the money have now been made by people who encountered Mr. Dorner on his final day, before he apparently killed himself with a gunshot to the head as law enforcement personnel surrounded him, an autopsy showed. But a large part of that reward has now been pulled off the table. Several donors have reneged on contributions they had pledged, declaring that the criteria for the reward had not been met because Mr. Dorner was found dead before he could be captured or put on trial. More contributors have indicated that they may soon come to the same conclusion.

The dwindling reward — coming now that the spotlight has moved away from the manhunt — looms not only as a potential embarrassment for Los Angeles officials, but also, experts said, as a threat to the effectiveness of what has long been one of the most common tools in law enforcement: the promise of money.

Law enforcement officials expressed worries that if part of the reward was not paid simply because the suspect died during the pursuit, it could leave members of the public less likely to come forward with information when future rewards are offered.