FDNY officials told Eyewitness News they believe the fire was started by a lithium-ion battery.
The Manhattan DA's Office on Thursday announced a criminal probe into the owner of 207 Dyckman Street.
On Monday, Eyewitness News learned that the twins, a boy and a girl, have also died from injuries suffered in the fire.
A memorial of flowers dedicated to Firefighter Andrew Cross, 27, was left outside of the Morrill Volunteer Fire Department.
In the wake of several deadly fires in recent days, the FDNY's commissioner has an urgent message for all New Yorkers that could help save lives.
FDNY investigators say the suspect deliberately discarded a lit cigarette into cardboard boxes, causing them to ignite.