It happened just before 4 p.m. on January 10 inside the apartment of 28-year-old Keishla Arroyo-Rodriguez.
According to police, the victim's ex-boyfriend, 32-year-old Elvyn Almodovar Rodriguez, shot and killed the woman while she was in the bathroom.
The victim's three children, ages 7, 3 and 5, were all inside the residence when she was killed.
The 7-year-old daughter of the victim told police she saw the suspect walk into the apartment, go into the bathroom, and then she heard several bangs, authorities said.
The child then reportedly looked under the bathroom door and called her father after she saw her mother lying on the floor.
An autopsy found that Arroyo-Rodriguez died of a single gunshot to the head.
Authorities say Arroyo-Rodriguez told her daughter's father that she'd been punched by Elvyn Almodovar Rodriguez just prior to the end of their relationship around Christmas.
"This murder of Keishla Arroyo-Rodriguez, a mother of small children, is a horrific ending to a domestic violence situation," said DA Steele.
The suspect turned himself in to police in York, Pennsylvania on Tuesday night.
He's facing a slew of charges including murder, burglary, theft by unlawful taking, endangering the welfare of children and other related offenses.
Anyone with information regarding this incident or Rodriguez's whereabouts is asked to contact the Norristown Police Department at 610-270-0977 or call the Montgomery County Detectives' Tip Line at 610-278-DOIT (3638).