The Associated Press quotes the source as saying police are also investigating the possibility that suspect Dennis L. Rader might be connected to yet another three murders.
At least one of those is said to have occurred after Kansas restored the death penalty in 1994.
Asked to comment, District Attorney Nola Foulston said, "Your information is patently false." But she refused to say whether Rader had made any confessions or whether investigators are looking into Rader's possible involvement in more unsolved killings. Police spokeswoman Janet Johnson also declined to comment specifically on the accuracy of the source's statements.
Rader is being held on $10 million bail in the deaths of ten people between 1974 and 1991. Police had long linked the BTK killer to eight murders, but added two more on Saturday after Rader's arrest and said at the time their investigation was continuing.
Now, the source said, police are looking whether Rader was responsible for the deaths of two Wichita State University students, as well as a woman who lived down the street from another known victim of BTK, the killer's self-coined nickname that stands for "Bind, Torture, Kill." It was unclear when the three slayings occurred, but the source said one of them took place while Kansas' death penalty was in effect.
Prosecutors had said initially they could not pursue the death penalty against Rader because the ten murders linked to BTK occurred before Kansas state law allowed capital punishment.
Rader, 59, could appear in court as early as Monday, when he will stand in front of a judge on video while prosecutors recite yet-to-be-filed criminal charges against him. The judge will also review Rader's bond and set a permanent amount.
The hearing could happen Monday but is more likely to be postponed until Tuesday, the district attorney's office said Sunday. It was unclear whether Rader had a lawyer.
Police were confident Rader's arrest last week would bring an end to 30 years of fear about the BTK strangler. But many Wichita area residents have been left with an unsettling feeling: that the killer was hidden among them all along.
Charlie Otero, whose parents and two siblings were BTK's first victims in 1974, said Sunday that he is "waiting with anticipation" to learn more about the DNA evidence that has been credited with helping crack the case.
Otero believes his family was targeted, although the rest of BTK's victims were likely chosen at random. He isn't sure why the family was targeted but said it's interesting that Rader and his father served in the Air Force at the same time in the 1960s. "I'm sure this will all come out during the trial," he said.
Rader, a married father of two, a Cub Scout leader and an active member of a Lutheran church, was anything but a recluse.
His job as a city code enforcement supervisor required daily contact with the public, and he even appeared on television in 2001 in his tan city uniform for a story on vicious dogs running loose in Park City.
As a city worker, Rader could be seen measuring grass in a front yard with a tape measure to see if it was too long, a neighbor said.
Before becoming a municipal employee, Rader worked for a home-security company, where he held several positions that allowed him access to customers' homes, including a role as installation manager. He worked for ADT Security Systems from 1974 to 1989 - the same time as a majority of the BTK killings.
Mike Tavares, who worked with Rader at ADT, described him as a "by-the-books" employee who would often draw diagrams of houses and personally make sure technicians installed systems correctly.
While Rader was known as a blunt person and rubbed some people the wrong way, it never struck co-workers as anything other than businesslike.
"I've spoken to some co-workers who were around then, and everybody is very numb," said Tavares, who left the company in 2001.
At his church and around town, many expressed shock that Rader was accused of being the BTK killer.
"Disbelief, absolute disbelief," said a tearful Carole Nelson, a member of Christ Lutheran Church in Park City, Kansas, where Rader was an usher and the president of the church council. "I never would have guessed in a million years."
The church's pastor, Michael Clark, said Rader's wife, Paula, was in a state of shock when he visited the family, who remained in seclusion Sunday.
"Her demeanor and voice indicated she was suffering," Clark said.
Bob Smyser, an usher at the church, says he has been at a loss as to how to explain this to his son, who saw Rader's arrest on the news.
"I'm not sure what to tell myself," said Smyser, in an interview with KAKE-TV. "He said 'Daddy, he tricked us didn't he?' And I guess that's as simple as it gets."
Another parishioner, Paul Carlstedt, recalls that only a few days ago, Rader brought spaghetti sauce and salad to a church supper, even though he was unable to attend himself.
"I was totally, totally, totally shocked that it could be anyone from this congregation or anyone I knew, especially this person. I mean this, alls we could say was, what!" Carlstedt told KAKE-TV. "I mean it just did not fit and does not fit the person that we know. "
"The guy that walked in here was not the face of evil," said Smyser, thinking back to the church supper.
A sense of relief is nonetheless palpable in many parts of Wichita, which has endured decades of unsolved murderers and taunting letters to the media.
"Hallelujah, praise the Lord," Gaylene Brown said over breakfast Sunday at Don's Restaurant, where Rader's face was a common sight.
Police have disclosed little about how they identified Rader as a suspect and have said they will not comment further on the case, but bits and pieces of the investigation have filtered out.
Kansas Gov. Kathleen Sebelius told The Associated Press that DNA evidence was the key to cracking the case. It was unclear whether BTK's letters helped lead to the arrest. Police have said they obtained semen from the crime scenes even though the killer did not sexually assault his victims.
KAKE-TV, citing sources it did not name, reported that DNA from Rader's daughter, Kerri, was instrumental in his capture, though KAKE anchor Larry Hatteberg said it did not appear the daughter turned in her father.
Parts of the profile released earlier by police seemed to match up. Investigators said they believed the killer was familiar with a professor at Wichita State University. Rader graduated from the university in 1979.
In the 1970s, Rader worked at a nearby Coleman camping gear plant where two of his victims were employed.