(CNN) A woman in Alaska charged with murdering her two children almost two years apart searched the internet for ways to kill -- and get away with it -- before calling for medical help for her baby daughter, police in Fairbanks said.

Stephany Lafountain called Fairbanks emergency services in November 2017, saying her 13-month-old child was not breathing. An hour earlier, police say, her internet searches had included "ways to suffocate," "ways to kill human with no proof," and "How to: Commit the Perfect Murder."

The child was rushed to a hospital but died four days later.

While investigating, police found that in September 2015, Lafountain also had called for help when her then 4-month-old child, also a girl, was not breathing. The death of that baby wasn't investigated as a homicide at the time, but the similarities in the two cases caused investigators to broaden their probe, the police said in a press release.

"This is just an unimaginable tragedy," Fairbanks Police Chief Eric Jewkes said at a press conference. "We have two infants, we have two babies, that were killed by their mother. How do we even imagine that?"

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