DOVER, Del.—A groundskeeper at a popular park here recently found a suspicious package stuck to the side of a large electrical box, just yards from a building where dozens of youngsters were about to play basketball.

Alarmed, the groundskeeper called the police, who mustered a bomb squad. From a safe distance, they sent a robot to get close-up video of the object—a small cylinder wrapped in camouflage tape. Then a technician in protective gear crept in with X-ray equipment.

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