Investigators are not expected to conclude their probe until Tuesday at the earliest, but two officials briefed on the probe tell ABC News that it was apparently caused by an electrical malfunction that occurred inside the apartment -- perhaps an appliance or device -- as opposed to inside the wall on an electrical line.
The fire was so extreme that it incinerated most of the usable evidence, and also, there was no working smoke alarm in the 50th-floor apartment.
The city medical examiner ruled Monday that the victim Todd Brassner, died from smoke inhalation.
Meanwhile, a City Councilman is pushing for new legislation to require sprinklers in older high-rise buildings that were previously exempted.
Legislation mandating sprinklers on residential floors was passed in 1999, but Trump Tower and other older buildings were grandfathered in, meaning there were no sprinklers installed where the fire occurred.
Councilman Robert Cornegy, Jr., hopes to toughen the policy and require Trump Tower and similar high-rises to install sprinklers.
It was what was known as the "Macaulay Culkin Fire" that pushed through the new regulations after the actor's family owned an apartment on West 60th Street where a fire broke out, killing four people.
Jerome Ross, a neighbor of one of those victims, became the chief advocate for sprinkler requirements. Ross said real estate developers lobbied against him because it was too costly to implement. Ross said leading that effort was Donald Trump.
"Mr. Trump, what's the price of your child's life?" Ross said. "What would you tell a real estate developer if your child has died and the real estate developer, you're confronting him, and he says to you, 'I couldn't afford sprinklers?' What would you say to him, Mr. Trump?"