Jenna Folwell, 19, has been arrested on suspicion of murder after the baby boy was found dead inside a black duffel bag in her apartment.
The grim discovery came after Folwell called police saying her son had been snatched at a park.
She claimed she was putting him in his car seat when someone came up from behind, put a bag over her head and ran off with him.
Police launched a huge search of the area where they found the baby - who has not been named at Folwell's apartment in Chandler, Arizona.
Officers then seized her phone and discovered more than 100 different web searches that ranged from "ways to die instantly" and "missing babies cases," police said.
She had also allegedly searched how long it takes for a baby to drown.
Police say Folwell then changed her story and admitted to faking the kidnapping and hiding her son's body in the bag.
It's alleged she told them she was taking a bath with her son and fainted. When she came back around, she said, her baby had drowned.
Investigators then asked Folwell about her web searches on her phone. Folwell then reportedly admitted to intentionally killing her son.
She said she decided to kill him by drowning because she didn't want to hear him cry, according to Chandler Police.
She reportedly claimed he was in the water for about a minute when she regretted her decision, pulled him out and tried to do CPR but it was too late.
Police said Folwell was booked into Maricopa County Jail on one count of first-degree murder.