According to KTLA, a witness told police of seeing Jacsun inside a blue 2006 PT Cruiser that night in south Los Angeles' Hyde Park neighborhood. Manson and Williams were allegedly trying to break into cars nearby.
The car burglaries were what resulted in Manson and Williams being arrested Jan. 3. Jacsun was not with them at that time, the news station reported.
It was not immediately clear why the baby was not reported missing for another three weeks.
The PT Cruiser, which police officials said was stolen, was found abandoned Feb. 8, the news station said.
As the investigation wore on, detectives began to suspect Manson and Williams were responsible for their son's disappearance and death. In a news release issued Monday, Culver City police officials alleged that the couple had put their son's body in a dumpster near the Crenshaw Mall in Los Angeles.
The district attorney offered a more complete version of the allegations.
According to prosecutors, Manson and Williams were staying in a South Los Angeles motel with their son on Dec. 31. The couple, who were doing drugs, allegedly found their son had died.
Instead of calling for help, the couple is accused of putting Jacsun's body in a suitcase and throwing it in the dumpster near the mall, the district attorney's news release said.
Jacsun’s body has not been found. His exact cause of death is unknown.
As of Monday, police personnel and volunteers were combing the El Sobrante Landville in Corona, where it is believed the baby's body ended up, Culver City police officials said.
"We continue to strategize and plan for the best and most efficient way to search the El Sobrante Landfill for the remains of Jacsun Manson," police officials said Wednesday. "We anticipate the search will resume in the near future."
Manson's sister told KTLA she and her brother were estranged "because of his lifestyle," but had spoken recently. In that conversation, he spoke as though Jacsun were dead, she told the news station.
If convicted, Manson and Williams each face a maximum sentence of 10 years in state prison, prosecutors said.