An explosion in Austin, Texas, left two men with serious injuries Sunday night. It was the fourth unexplained explosion in the city this month, and police believe the latest blast is related to the earlier ones, which killed two people. The fourth blast occurred on a roadside in an upscale neighborhood, while the earlier package-bombs exploded on the doorsteps of homes. The victims in the latest explosion, both in their 20s, were walking or riding bicycles, and Austin Police Chief Brian Manley said it was "very possible" that the device went off when one of them touched a tripwire. "We are dealing with a serial bomber," he added at a news conference Monday morning.

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