Ocean County Prosecutor Joseph Coronato said the cigarette was discarded carelessly in the designated smoking area at the Mariner's Cove Motor Inn, where guests included some people who were previously displaced by Superstorm Sandy in Oct. 2012.
The prosecutor did not immediately say whether authorities know who discarded the cigarette or whether it was one of those killed or injured in the fire.
"This is only the beginning of an intensive investigation aimed at answering the many questions surrounding all the circumstances that contributed to the tragic deaths, injuries and loss of property," Coronato said.
He said the investigation was aided by video recovered from the Point Pleasant Beach motel's heavily damaged surveillance system. Investigators also relied upon pre- and post-fire photos and interviews with guests and motel management.
Coronato identified the victims as 45-year-old John Alberti, of Keansburg; 20-year-old Paulo Martins, of South River; 52-year-old Harold Ford, of Neptune; and 66-year-old Albert Sutton, of Mount Laurel. Autopsies by the Ocean County medical examiner's office determined that all four died from smoke inhalation.
Eight other people were injured in the fire. They include Keri Anderson, 42, of Keansburg, who had sought refuge in a shower with the water running while waiting to be rescued. Firefighters rescued her from a bathroom in the rear of the motel after dousing flames over her head and helping her down a ladder to safety. She remained in critical condition in the burn unit of St. Barnabas Medical Center.
Also in that hospital's burn unit was Melanie Deieso, whose hometown was not available. The 22-year-old suffered second-degree burns to her legs.
The investigation was also expected to look into whether smoke detectors or alarms were present at the motel and whether they were working properly. Survivors gave conflicting accounts to The Associated Press on Friday of whether they heard any alarms when the fire broke out.