"I think it was definitely orchestrated," Steve Powell, Susan's father-in-law, said on NBC's "Today" show this morning.
Steve Powell points to journals his daughter-in-law allegedly kept before she went missing in which he claims she was "out doing something with another guy and was going to run off with somebody else," Powell said.
On a website run by the Powells, they point to several parallels and time lines that connect the two missing persons, The Salt Lake Tribune reports.
"Susan Powell and Steven Koecher are the same age," reads an entry on the website. "Susan Powell and Steven Koecher are members of the same Mormon faith (Latter-Day-Saint, LDS Church). Susan Powell and Steven Koecher were both in West Valley City, Utah, during the week of their parallel disappearances. Susan Powell and Steven Koecher disappeared at the same time."
The Powells claim the new theory has been shared with the FBI and police in West Valley City.
According to Susan's father, the authorities have looked into the latest allegations and do not consider them relevant to the case.
"That is ridiculous," Chuck Cox told "Today" this morning. "The police are aware of this ... and we know this has been looked into and it just does not fit. My daughter loved her children and she didn't leave of her own accord." He added, "I know that did not happen, that she did not take off with anybody. She is dedicated to her family and she is missing ... the story Josh Powell has come up with is just not believable."
Koecher's father, Rolf Koecher, also doubts the two cases are connected.
"We don't think there's a link," Koecher told Salt Lake City's KSL News Radio. "We're not mad at [the Powells], but we don't agree with what they're saying."
Susan was reported missing by her family on Dec. 7 when she failed to show up for her job as a stockbroker at Wells Fargo Financial. Powell told police he had been camping with their two children, ages 2 and 4, and had last seen his wife around midnight.
Suspicious of his story, investigators named Powell a "person of interest" in his wife's disappearance. Several search warrants were served, including one to draw biological samples from him. Police also impounded the family van.
Joshua Powell and his two children have since moved back to his hometown of Puyallup, Wash.
Afterward, Powell's sister, Jennifer Graves, appeared on ABC's "Good Morning America" and accused her father and brother of making the story up.
"They want to make themselves look better, or less guilty, I should say," Graves told "Good Morning America."
No one has been charged in Susan Cox Powell's disappearance, and police say the case remains an active investigation.