Pills were found in the stomachs of Leisa Jones and the teen -- both considered suspects in the gruesome deaths -- during autopsies last week, said the officials, who spoke on condition of anonymity because further tests on their causes of death were pending.
The preliminary findings deepened the mystery surrounding a case that was briefly believed to be an accidental fire before becoming a homicide investigation.
The New York Police Department said in a statement Monday that there still was "no final determination as to who was responsible."
Firefighters responding early Thursday at the family's Staten Island home found the charred bodies of Jones and two daughters, ages 7 and 10, in a front room, and that of her 14-year-old son, C.J., slumped over a bed in a back bedroom. A 2-year-old son pulled out alive died later at the hospital of smoke inhalation.
Later that day, police said that the throats of the two girls and C.J. had been slashed. They also said that a razor had been found under C.J.'s body and that he had a history of playing with fire, making them theorize he might have killed his family, set the blaze and cut his own throat.
Since then, police have determined that a badly damaged, fragmented note with the words "am sorry" that was found in an another room was written by Jones. The significance of the note was unclear, but that discovery -- combined with the initial drug evidence -- has led to suspicions she may have killed her children.
A document search turned up three legal filings connected to Jones.
In New York, she was named as the debtor in a filing in civil court. The filing date was in February, and the amount was for $6,580.
In Washington, D.C., she was listed as the debtor in landlord/tenant dispute that was filed in November 2008. She was also named as the debtor in a July 2000 filing in which the creditor was the now-closed D.C. General Hospital. The amount was $2355.83.
Jones' mother, Marcia Anderson, declined to comment Monday when contacted by The Associated Press.
Anderson told The New York Times, "My grandson is not a killer and my daughter is not a killer. She could never harm her babies."