Nine children were dead due to an outbreak of adenovirus at a rehabilitation center in the U.S. state of New Jersey, officials announced Sunday.

The New Jersey Department of Health said in a news release that "another medically fragile child" who had a confirmed case of adenovirus at the Wanaque Center for Nursing and Rehabilitation died Saturday night. The child had become ill before Oct. 22, according to officials.

Adenovirus usually causes mild illnesses, but the health department said this outbreak was particularly severe because it affected medically fragile children with severely compromised immune systems.

The diagnosis brings the total number of cases at the facility to 25. The facility has been instructed not to admit any new patients until the outbreak ends.

"This strain has been particularly associated with disease in communal living facilities," the department said in an earlier statement. "The combination of a worse strain of adenovirus together with a fragile population has led to a more severe outbreak."

A health department team is at the facility, and an investigation is underway.