(CNN) Twenty-five people have been hospitalized in the same area of Brooklyn after having negative reactions from what's believed to be synthetic marijuana known as K2, according to Lt. Paul Ng with the New York Police Department.

There have been no fatalities, Ng said Saturday night. According to police, the hospitalizations began Saturday evening and continued over the course of several hours.

Among those hospitalized, 15 people were treated at NYC Health + Hospitals/Woodhull in Brooklyn. As of Sunday morning, about half of them had been released and the others were in the emergency room "under observation," hospital spokeswoman Ana Marengo said.

Police are investigating drug activity in the area of Brooklyn where the illnesses were centered, and attempting to locate any more victims and also drug dealers, Ng said.

He said while police cannot say for certain that the synthetic drug caused the illnesses, that is what investigators suspected.