Police have concluded that a couple spotted at a gas station in Illinois is not the duo the FBI is calling "the most wanted couple in America," FOX News has learned.

Authorities believe that Richard Davis, 41, and Dena Riley, 39, may have fled Missouri after allegedly raping, beating and strangling 41-year-old Marsha Spicer and burying her in a shallow grave. Police found a videotape of the activities leading up to Spicer's death, and her actual killing, in the couple's apartment.

On Thursday, reports surfaced that the pair may have been seen at a Columbia, Ill., Shell station on Monday, but Independence, Mo., police have told FOX News that the pair seen in the video is not Davis and Riley.

There was also a report that the couple may have also been seen at a motel in adjacent Waterloo, Ill.

"At this point they only have each other and I don't think they'll separate," said Tim McCorkle, the police chief of Bates City, Mo., where investigators focused a search Thursday for more clues and possibly bodies.

"It's conceivable they're outside the Midwest," added Michael Sanders, the prosecuting attorney in Jackson County, Mo.

Local authorities combed a half-mile rural area in Bates City, Mo., near where Spicer's nude body was found on May 15. The search only turned up a few deer bones.

The Independence, Mo., couple has been on the run since May 19, when police discovered the videotape.

"These are two of the most violent people that I think we have that are currently at large within the state of Missouri," Sanders said. "On top of that, I think they're becoming increasingly desperate ... they don't have large amounts of cash.

"As this goes on, I think they're going to get more and more dangerous, more and more desperate, and that's why it's critical we get them into custody as soon as possible so other people aren't injured."

"We’ve had some pretty heinous crimes around here, but nothing of this particular nature," FBI Special Agent Jeff Lanza told FOX News.

Spicer had no idea what the couple was capable of, according to the victim's older sister, Jackie Schumacher.

"I want them to remember everyday of their life what they're doing to all these people," Schumacher said. "People don't deserve this ... people don't deserve what they did."

Kansas City resident Sherry Ballew fears that her daughter might also have fallen victim to the pair after seeing a still picture of who she says is her daughter, Michelle Ricci, taken from the videotape. Ballew hasn't seen her 36-year-old daughter since a few days after Ricci met the couple.

The wanted couple was sighted Sunday in Perryville, Mo., at the Hilltop Apartment Complex.

The Missouri State Highway Patrol interviewed a woman who about the couple's disappearance at the apartment complex on Sunday. Police later learned the couple had been hiding in the back room of the woman's apartment at the Perryville complex and escaped after officers left. The woman, Susan Summers, has been charged with two counts of hindering prosecution.

Lanza believes the fugitives are unlikely to split up.

"They’ve committed the crimes together, they’ve made this videotape together, it’s unlikely they would separate," he said. "Although we’re prepared for all types of contingencies here, the likelihood is they’ll be together."

The Kansas City Star reported Thursday that the couple worked at a Kansas City fabricating business. Davis worked as a janitor and Riley worked part-time in light assembly.

Click here to read the Kansas City Star story.

Davis and Riley are each charged with first-degree murder, first-degree assault, kidnapping, forcible rape and two counts of forcible sodomy.

Spicer is survived by two children and two grandchildren.

FOX News' Sara Bonisteel and Jeff Goldblatt, and The Associated Press contributed to this report.