San Francisco Police arrested 33-year-old Mark Dennis -- not long after they say he mowed down five pedestrians with his van, then took off.
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It happened in the Dogpatch neighborhood at 24th and Illinois Street.
"He backed up really fast, the van driver, turned around and just went head on through the men like they were bowling pins," said witness Shannon Gilcrease.
The five men were sent to the hospital. One of them died. Another is in critical condition. It's unclear what triggered the incident, but witnesses say Dennis was acting erratically before he struck the men down -- banging on his windows, throwing things, even getting out of the van and chasing them with an ax.
"The guy in the van was just crazy and took it to another level and got out and went after the guys with an ax and they just took off their belts and were lashing at him," said another witness.
VIDEO: 1 killed in San Francisco hit-and-run crash
1 killed in San Francisco hit-and-run crash in city's Dogpatch
Dennis has been booked on suspicion of murder and assault with a deadly weapon. Witnesses say they will never forget the sheer horror of what they saw.
"I froze just looking at it because I thought, how did he just do that, like with no hesitation," said Gilcrease.
It's now up to investigators to try and get some answers, how it escalated so quickly and why.