The majority of the deaths were individuals who got caught in their vehicles by flooding and were overtaken by the water.
Four residents of an apartment complex in Elizabeth were found dead in the aftermath, according to a city spokesperson.
Oakwood Plaza, located across from the Elizabeth Fire Department headquarters, was inundated with eight feet of water, destroying fire equipment and apparatus.
Rescuers were going door to door through the complex to check for additional victims.
Mayor Chris Bollage said the city is assisting some 300 displaced residents.
A person in a car was killed after becoming trapped in six feet of floodwaters in Passaic, after the Passaic River flooded, prompting evacuations of 60 residents to City Hall.
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According to a preliminary investigation, firefighters and rescue crews pulled 25-year-old Jonathan Jalil and his 66-year-old mother Isable Jalil from the submerged car at Passaic Avenue and Lackawanna Place in what the mayor described as a heroic effort, but they were unable to save their 70-year-old father and husband, Luis Jalil-Solarzario, who drowned.
Rescue workers are searching for two others who may have been swept away.
"Unfortunately we have confirmed the loss of a life within the city of Passaic and have unconfirmed reports of additional lives that have been lost," Mayor Hector Lora said.
Eyewitness News spoke with Lora about the deadly flooding
Describing the rescue effort, Lora said the car was overtaken by water and "firefighters were being dragged down under the vehicle and the vehicle began to float away."
"We are now dealing with the aftermath in our city," he said. "We see vehicles that were abandoned, so many individuals that were injured had to leave property behind."
Somerset County confirmed two fatalities from the flooding in two separate incidents in Hillsborough as well as two bodies recovered in Bridgewater, and Hunderton County officials said one person was found dead in a heavily damaged pickup truck that was discovered at daylight in Milford.
Another person was found dead in Maplewood after venturing out into the storm. Authorities said it appeared he had been attempting to remove debris from drains in the area when he was suddenly swept up into dangerous flood waters.
Water rescues were underway with people sitting on their roofs in Cranford, where entire neighborhoods were under water and rescue crews were using boats to aid people in their evacuation.
Governor Phil Murphy declared a state of emergency in all of New Jersey's 21 counties and urged people to stay off the flooded roads.
"An extraordinary, sadly tragic, historic 24 hours in New Jersey," he said. "There's no other way to put it."
The governor said the state will use every resource at its disposal to ensure the safety of New Jerseyans.
"I'll close with a pretty obvious statement, but the world is changing, right?" he said. "These storms are coming in more frequently...they're coming in with more intensity and as it relates to our infrastructure, our resiliency, our whole mindset, the whole playbook that we use, we have got to leap forward and get out ahead of this."
Paterson Public Schools and several other districts announced they were closed or opening late Thursday, and New Jersey state offices also had a three-hour delayed opening.
All NJ Transit rail service on the Northeast Corridor, with the exception of the Atlantic City Rail Line, was suspended due to weather-related issues.
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