An investigation is now underway to determine the cause of a five-alarm fire in the Bronx on Thursday that left at least 12 people dead.

Four others are in critical condition.

The Fire Department says the flames broke out on the first floor of a five story building around 7 p.m. Thursday on 2363 Prospect Avenue in Belmont.

A one-year-old child is the youngest death - the oldest is 50 years old. 

Mayor Bill de Blasio and Fire Commissioner Daniel Nigro called the tragedy historic while speaking to reporters at the scene Thursday.

"Tonight here in the Bronx there are families that have been torn apart. This is the worst fire tragedy we have seen in this cityt in at least a quarter century," De Blasio said.

"In a department that's certainly no stranger to tragedy, we're shocked by this loss," Nigro added.

Nearly 200 firefighters battled the blaze at the five-story, walk-up building.

Temperatures in the teens with wind chill in single digits added to the difficulty of their battle.