When deputies arrived, they found Mammone lying in an intersection with "severe injuries," authorities said.
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Dr. Michael Mammone, 58, was killed Wednesday by a motorist who struck his bicycle and then attacked him, authorities said. (Providence Mission Hospital Laguna Beach)
Witnesses said the suspect struck the bicyclist and proceeded to stab the victim at least once in the back, FOX Los Angeles reported. The suspect then allegedly pulled out a gun and fired multiple rounds before he was disarmed by two passersby.
Smith was detained by bystanders until authorities arrived. He was arrested on suspicion of murder, the sheriff's department said. He is slated to appear in court Friday, according to jail records.
A knife believed to have been used in the attack was found at the scene.
Investigators said there was no known connection between Smith and Mammone. Mammone was a doctor at the Providence Mission Hospital Laguna Beach, where he practiced emergency medicine.
The scene where a doctor was killed by a motorist Wednesday in what authorities said was a violent attack. (KTTV)
"We are stunned by this devastating tragedy," a hospital statement to Fox News Digital read. "The entire Mission Hospital family is grieving over the loss of an incredible physician and friend. We will honor Dr. Mammone’s dedication to our community and passion for medicine by continuing to provide exceptional care."