BY CHRIS HARRIS
Illinois authorities tell PEOPLE they cannot say with any certainty who fathered the infant delivered two weeks ago by a 13-year-old girl who has been missing since Sunday, when police allege her 39-year-old stepfather kidnapped her.
But "once we are able to find out the paternity of the child, there will no doubt be charges of criminal sexual abuse filed against that individual," says Greenville, Illinois, Police Chief Lou Lorton.
Police are still searching for Christopher Derleth, who they say vanished along with his stepdaughter, Katherine Derleth, and the girl's infant child, who is also named Christopher.
According to police, Christopher's van was spotted just south of Charleston, West Virginia, late Sunday, and an AMBER Alert was issued. But the three haven't been seen since.
Investigators have filed charges against Christopher for violating a protective order barring him from having contact with Katherine, and officials indicate they plan to add kidnapping and child abduction charges.
Police are not discussing what prompted the alleged kidnapping but have said they believe the abduction was planned.
As a result of that probe, which focused on her stepfather as well as another man she had been living with, the state removed Katherine and her son from her home and placed them with a foster family, Lorton says.
A protective order was also issued, barring Christopher from contact with the teen, according to authorities.
"The original complaint stemmed from the fact that there was a 13-year-old that was pregnant and there were, at the time, two individuals living with her in the home," Lorton tells PEOPLE. "We obviously don't know who the father of the child is, but that's what launched the investigation."
According to Lorton, detectives were working to determine the child's paternity before Katherine and her son were abducted.
"We are still investigating, and we are fairly certain Katherine and her stepfather are out there," Lorton says.
The state had also looked into a 2015 complaint involving Katherine but had insufficient evidence to continue with a full-scale investigation, authorities said.
Katherine's mother, Jennifer, told KSDK she is praying for her daugher. She said the teen loves cooking, drawing and music.
Jennifer declined to discuss Christopher's alleged kidnapping, citing legal reasons, according to the station.
"To Katherine, if you're listening to this, know that your sister and your family all love you very much," Jennifer said. "Stay strong, we're praying for you and have faith that you'll be in my arms again."
Let us join her in that prayer.
Police are asking that anyone who sees Katherine, her baby or Christopher immediately call 911.