An investigation is underway after a woman and two children died in a house fire in North Philadelphia early Tuesday morning.

The fire broke out around 1 a.m. at a home in the 3400 block of Hope Street. Three people — 46-year-old Nidre Annette Ayala, 6-year-old Kataleya Garcia and 5-year-old Sebastian Rodriguez-DeJesus — were found by crews on the second floor of the home and taken to area hospitals, where they were pronounced dead. 

The children's great-grandfather said the children were siblings and were staying with Ayala when the fire happened. Their mother was out at work, the relative said. Relatives told CBS News Philadelphia that Ayala was a second cousin.

"This is a very, very difficult morning," said Fire Commissioner Jeffrey Thompson, who added that it took about four minutes for firefighters to get to the home after the initial call.

Deputy Fire Chief Frank Hannan said crews found heavy smoke on the first and second floors when they got to the home. 

"Two engines, two ladders went into service," he said. "They went into service and did a search and rescue. We found three occupants. They removed the occupants."

Thompson confirmed that firefighters did not find any working smoke alarms inside the home and said there were no reports of smoke alarms going off.

"If you do not have smoke alarms, please call 311 and we will come out and install one for you," Thompson said. 

Officials have not yet said what caused the fire. The blaze was placed under control around 1:30 a.m.

A neighbor is now taking care of a kitten that was saved from the fire.

Firefighters plan to return to this neighborhood in the coming days to hand out smoke detectors.