The girlfriend, identified by a Des Moines police spokesman as Alyssa Chang, allegedly brought the 2-year-old boy to her brother's house to have him babysit the child.
On Tuesday night, Chang told her brother, John Truong, that the toddler was her boyfriend's son, police said. Truong said he didn't realize anything was awry until Wednesday morning when he saw an Amber Alert about the alleged kidnapping on social media.
"It was my sister who had asked me as a favor to watch her boyfriend's son. And that's how he came and was dropped off.”
Police say the incident unfolded early Tuesday afternoon in Des Moines when the child’s grandmother Nguyen, 65, and Chang, 46, arrived at a home and allegedly abducted both the child and his mother, whose name has not been released.
In order to subdue the child's mother, police say, either Nguyen or Chang allegedly struck her with a stun gun before trying her up and driving her and the toddler to another home in a town 20 miles away in the town of Fife, Des Moines police Sgt. Mike Graddon told ABC News.
Once they arrived at the home in Fife, the women allegedly retied the child’s mother in the garage and took the child into the house. Police say the child’s mother was able to get out of the ties but not into the house, so she ran to a neighbor’s home.
Rather than immediately call police, however, investigators say she called a relative. It was not until about 9:00 p.m. when police were notified and they sent out an Amber alert.
When the child’s mother spoke to investigators, she was able to identify both of the suspects, authorities say, but the case is still under investigation.
"There’s a lot of family dynamics involved," Sgt. Graddon told ABC News. "As far as why, we can only speculate."
Police were only able to locate the child the next morning, however, when Chang’s brother called after seeing the Amber Alert online.
After Truong contacted police, they were able to secure the boy, who was unharmed. From there, they were able to locate Chang and the car used in the alleged abduction at the Fife home that the child’s mother had directed police to during her initial 911 call.
The boy’s father, Kang Tran, met with police Wednesday morning before the boy was found, police said. After hours of interviews, he was not charged in connection to the incident, police say.
The boy was reunited with his mother and his father Wednesday afternoon.
Chang was taken into police custody and Nguyen turned herself in later Wednesday. Police say both face three felony charges: kidnapping, felony assault, and unlawful imprisonment.
Police Sgt. Graddon told ABC News Chang is still in police custody but Nguyen was fingerprinted and released for medical reasons.
"We’re still trying to make sense of this whole thing," he said.
ABC News has been unable to reach the suspects. Police don’t know whether they have lawyers and a court appearance has not been scheduled.