New York City police investigators said Sunday that they were looking at whether a driver suffered "some sort of seizure" before he smashed into a group of trick-or-treaters, killing three people.

The New York Post reported that the driver, identified as 52-year-old Howard Unger was seen shaking violently just before his car jumped the curb and had recently been placed on anti-seizure medication.

"He was driving, and he might have started his seizures," a police source told the Post, "and he bumped the car in front of him." Members of Unger's family told the paper they were unaware of him having any medical problems.

Unger's black Dodge Charger plowed into the pedestrians on a sidewalk and then smashed through a fence in front of a home, police said. The driver was taken to the hospital in stable condition. No charges had been announced as of Sunday afternoon, police said.

A 65-year-old grandfather, Louis Perez, suffered severe head trauma and died at the scene, police said. His granddaughter, 10-year-old Nyanna Aquil, was pronounced dead at a hospital. The girl's 3-year-old sister was also hospitalized.

Another man, 24-year-old Kristian Leka, was also killed. His 9-year-old sister and a 21-year-old female friend were also injured but not critically.

Witnesses described hearing a loud boom, followed by screaming and crying, then seeing a trail of mangled bodies in crumpled, bloodied costumes.

"I saw a torso on the sidewalk. I didn't know if it was a Halloween dummy or a real person," neighbor Fabio Cotza told the New York Post. "I just grabbed a whole bunch of towels and ran outside."

Hours after the crash, neighbors gathered for a small candlelight vigil to remember the victims.

Mayor Bill de Blasio said authorities "will leave no stone unturned" in the investigation and offered prayers to the victims and their families.

"We do not accept tragedies like this as inevitable," he said. "This could be any of our families. Each of us must contribute to making this a city where everyone, especially children, can walk our streets safely."

The Associated Press contributed to this report.

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