Firefighters began evacuating The Station at Lyndhurst Apartments on Chubb Avenue around 11:15 p.m. Thursday, residents said.
They were able to return to their homes around 2:30 a.m., after the complex was examined and ventilated, Deputy Fire Commissioner Paul Haggerty said.
Local authorities and building management were planning to resume their search for the cause while a group of firefighters remained on scene throughout the night to conduct routine air samples, Haggerty told Daily Voice.
A Bergen County HazMat Unit joined Lyndhurst police, firefighters, EMS and emergency management professionals at the scene, as did a representative from the Bergen County EMS Task Force.
Rutherford firefighters provided an engine and North Arlington a ladder.
Click here to follow Daily Voice Rutherford and receive free news updates.