"This is an active investigation of an outbreak of adenovirus so it is possible that lab tests will confirm additional cases", the state's health department said in a statement.
"This is a tragic situation, and our thoughts are with the families who are grieving right now", Health Commissioner Dr. Shereef Elnahal said.
The department says there have been eight pediatric deaths associated with the outbreak and at least one young adult.
The nine children who died in the outbreak that began September 26 range in age from toddler to young adult, with a lot of them under age 18.
United States health officials have confirmed another case of a virus that has left eight children dead at a paediatric rehabilitation centre.
The illness, identified as adenovirus 7, poses the most significant risk to patients with weakened immune systems or existing respiratory and cardiac disease.
The children affected were previously described in a health department statement as "medically fragile children with severely compromised immune systems". It is believed that the individuals who became ill contracted the virus between September 26 and October 22.
"Deeply saddened to report a 9th death from the Wanaque outbreak".
This 1981 electron microscope image made available by the Centers for Disease Control and Prevention shows a group of adenovirus virions.
All nine were in the pediatric unit of the Wanaque Center for Nursing & Rehabilitation in Haskell, Passaic County.
The viruses, unlike the flu, are not seasonal and can cause illness throughout the year. Type 7 is most commonly associated with acute respiratory disease, according to the CDC. Symptoms include the common cold, sore throat, bronchitis, pneumonia, diarrhea, pink eye, and fever and bladder infections.
Still, Schaffner doesn't think people need to be anxious about adenoviruses.