Sharon Hill police say the person who pulled the trigger was 23-year-old Raquill Holland, who then fatally shot himself.
According to investigators the couple was found dead in their apartment by a family member who came to check on them.
Their son, Raquill Jr., survived two gunshot wounds to the torso and leg, police said.
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"That day, when the officers got there, he was actually on the ground crawling around on the floor, not crying," said Chief Richard Herron with the Sharon Hill Police Department.
Police were called to the house once before on the 100 block of Laurel Road in Sharon Hill, back in October 2018, for a domestic dispute.
Investigators said after the fatal shooting they discovered deep-rooted issues.
"We learned from interviewing family members that there had been domestic problems where he has threatened her in the past," said Herron.
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The baby was taken to Children's Hospital of Philadelphia and is in stable condition.
Officials said he is under the care of Children and Youth Services as they look for surviving family members to place him with.
If you or someone you know is struggling with domestic violence, visit this website for more information: https://www.thehotline.org or call the National Domestic Violence Hotline at 18007997233.