John Baker, 54, suffered the seizure in the Passaic County Jail's intake unit Tuesday night, said Bill Maer, the spokesman for Sheriff Richard H. Bernik.
Baker was taken to St. Joseph's Regional Medical Center, where he was pronounced dead a little over an hour after the 10:08 p.m. call, he said.
The cause of death was being investigated.
Police had charged Baker earlier with five counts of aggravated assault for setting his pit bull loose on police during a Ringwood traffic stop, forcing them to shoot the dog, authorities said.
Baker had been "weaving all over the road" in Wanaque while towing luxury boats Monday afternoon, one responder said.
He initially refused to stop, then finally pulled over on Greenwood Lake Turnpike (County Road 511) just north of Skyline Drive, authorities said.
Police from Ringwood and Wanaque said they asked Baker to keep his pit bull contained in the cab, but he let the dog loose.
Two of them -- one of them a former Passaic County sheriff's K9 officer -- fired to stop the nearly 100-pound dog, who didn't survive the injuries after charging the five officers, they said.
The first shot didn't stop the pit, so a few additional rounds were fired, Ringwood Police Chief Joseph Walker said.
The incident upset the officers, he said, noting: "They're animal lovers."
Ringwood police charged Baker with the aggravated assault counts and Wanaque police hit him with DWI, reckless driving and eluding, among other offenses.
He was sent to the Passaic County Jail to await a detention hearing.
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