The horrific murder-suicide that ended in an arson on Staten Island was committed by the deranged mom, who slit three of her kids’ throats before she killed herself and her baby in the blaze, law-enforcement sources said yesterday.

Cops had at first suspected Leisa Jones’ oldest

child, 14-year-old son C.J. Romoy, of the heinous act.

But autopsy results now show that neither he nor his two sisters, ages 7 and 10, suffered any smoke inhalation, meaning they were dead before the fire was set in their Nicholas Avenue home, the sources said.

All three children were found with their throats slashed by an old-fashioned barber’s straight razor, sources said.

Meanwhile, Jones and her youngest son, Jermaine, 2, both died from smoke inhalation, sources said — leaving investigators with conclusive proof that the unbalanced mother murdered her three older children first, then set the blaze, trapping herself and her baby.

Both Jones and C.J. still had undigested pills in their stomachs, a source said. Some investigators speculated that this was because she may have needed to drug the teen before she could kill him, and then took pills herself. Sources could not identify the drugs.

Cops yesterday confirmed that a singed note found amid the debris with the words “am sorry” was written by the mother, as The Post reported yesterday.

The deeply disturbed Jones had allegedly told a pal several months ago that she thought about killing the kids and torching the house.

C.J.’s dad, Earlston Raymond, said the friend told him Jones told her: “At times, I feel like killing the kids and burning the house down and killing myself.”

“[Jones] made that statement to more than one person,” said the distraught dad, who lives in Jamaica and added that he learned of his ex’s threats only after the tragedy.

Jones and her children moved to Staten Island several years ago from Washington, DC. Raymond described his ex as hot-tempered and filled with negative thoughts. He said she was trying to sour the relationship between him and his son.

“I’m sorry I signed the paper to send C.J. back to Washington,” Raymond said, adding that he never believed his son committed the gruesome crime.

The bodies of Jones, 30, and her children were found Thursday after the 4 a.m. fire. Investigators had originally set their sights on CJ because he was known around the neighborhood for his bad habit of playing with fire.