N E W Y O R K, April 10 — In an exclusive Good Morning America interview, Steve Thomas, one of the lead investigators in the JonBenét Ramsey case, talks to ABCNEWS' Elizabeth Vargas about who he thinks the killer is.

In his new book JonBenét: Inside the Ramsey Murder Investigation, Thomas talks about the 18 months he spent on the Ramsey case and what he believes to be the truth. What follows is a transcript of their conversation as it aired on Good Morning America.

Steve Thomas: Strip away the the pageants and the outfits, this was a great little kid. A great kid who we had a tremendous obligation to and who we failed.

ABCNEWS' Elizabeth Vargas: (VO) Meet Steve Thomas, once a lead detective on the case devoted to finding the killer of JonBenét Ramsey. He finally quit the case, frustrated with an investigation he says was doomed from the start.

Thomas: I gave up a career, my livelihood, what I love to do in essence, to blow the whistle on how flawed this system inside and out was in Boulder.

Vargas: For years, you have believed that the Ramsey's were involved or guilty in the murder of their child. Five pages into your book you reveal which one you think killed JonBenét Ramsey. I want to read to you this one portion. "There are only two possible answers. One is that an intruder crept into the house, killed JonBenét in a botched kidnapping attempt while the family slept, leaving behind what has been called the War and Peace of ransom notes, and then disappeared. The other scenario is that the little girl was killed by a family member, who I believe to have been her panicked mother, Patsy Ramsey, and that her father, John Ramsey, opted to protect his wife in the investigation that followed." Is it ethical for you to write a book like this?

Thomas: Well, that's a fair criticism, and let me respond to that. As a detective told me recently, 'Hey Steve, if you had just kept your mouth shut this whole thing would have gone away.' I think the sins of this case propelled me and compelled me to tell the story, and I think it was a story that was worth telling.

Vargas: (VO) Thomas says because of the secrecy of the grand jury and because so many questions remained unanswered by the Ramseys, he decided to take a bold step forward.
(To Thomas) Do you think Patsy Ramsey killed her daughter?

Thomas: I do. People want to have this inherent belief that a parent could not do something like this. That some three-eyed, six-fingered, stub-footed intruder came, some monster came in the house and did this. And believe me, Elizabeth, the detectives certainly were looking far and wide for this person. But that was not where the evidence was leading.

Vargas: Let's go through that evidence, then. What is it you think implicates Patsy Ramsey? What makes you believe that Patsy Ramsey is the killer?

Thomas: I think the devil's in the details. Brick by brick by brick I think a fairly compelling argument is made. We've been accused of not investigating other suspects in this case. That is entirely not true. I cannot tell you how much time, and time on the road, I spent chasing leads far and wide, interviewing suspects, witnesses, potential suspects.

Vargas: (VO) Thomas says investigators interviewed 590 people, investigated and cleared more than 100 suspects and collected 1,058 pieces of evidence. But the trail always led back to one place. That epic ransom note. The note asked for $118,000 and claimed to be from a small foreign faction and was signed by the mysterious S.B.T.C. Three pages of crucial clues. Thomas says they checked handwriting samples from 73 potential suspects, but only one person could not be ruled out as the author.

Thomas: And that one person happens to be Patsy Ramsey.

Vargas: (VO) Thomas says there are several reasons why. Among them is handwriting.

Thomas: And in Patsy's pre-homicide writings, she consistently used what we called the lower-case manuscript a. In the ransom note, almost exclusively, the lower-case manuscript a was used, I think, 98 percent of the time in the ransom note. That — so what, you might say, but what was telling was that after the Ramsey's were given a copy of the ransom note after the killing, Patsy Ramsey stopped using her pre-homicide habit of this lower-case manuscript a, and began a lower-case cursive a.

Vargas: So you're saying that in the ransom note this style of the letter a was used, and that in all Patsy's writing samples prior to the murder, she used an a like this most of the time when writing?

Thomas: Right.

Vargas: But after the murder she changed her writing style to use this kind of a, so that her writing samples after the murder wouldn't match the ransom note.

Vargas: (VO) That analysis is from a leading expert used by the FBI.

Thomas: Out of all the suspects that we investigated, Patsy Ramsey was the only one that he felt was consciously changing handwriting and handwriting habits after the crime.

Vargas: (VO) Thomas says there are still more similarities between Patsy Ramsey's personal letters and the ransom note.

Thomas: The indentation, for example, on the sign-off. "It's up to you now John, " exclamation point. Indented, "Victory"! Indented, "S.B.T.C." We were able to find in prior writings of Patsy, for example, how she would almost consistently sign off with the exclamation point, and then a double indented closure, to the point you could almost overlay the ransom note onto some of Patsy's previous writings, and the indentation I found remarkable.

Vargas: (VO) And in Patsy Ramsey's previous writings there was something more. Thomas notes her penchant for acronyms, like B.V.F.M.F.A., that's Barbara V. Furney, Master of Fine Arts, which she called her friend on her Christmas card. She signed the card P.P.R.B.S.J. Patsy Paul Ramsey, Bachelor of Science and Journalism. Patsy Ramsey denies she wrote the ransom note, which was signed with the mysterious initials S.B.T.C.

Vargas: In addition, you point out that the ransom note comes from the notepad in the house that was Patsy Ramsey's notepad.

Thomas: Well, that was a remarkable find to us.

Vargas: (VO) John Ramsey had given the pad to detectives when asked for samples of Patsy's handwriting. That was when Boulder police discovered a surprise. What appeared to be a practice ransom note on the 26th page.

Thomas: The first 12 pages of a tablet, a legal tablet not unlike this, had been previously removed.

Thomas: The next four pages, which would have been pages 13 through 16, were Patsy's doodles and handwriting. Lists and luncheon plans and so forth.

Thomas: Seventeen through 25 were missing from the tablet. We never recovered 17 through 25. Page 26 was what's come to be known as the practice note. But what's remarkable about page 26 was that somebody on page 25, with what we believe to have been a felt-tipped instrument, wrote approximately three-quarters of the way down on the page, leaving bleed-through on to 26, the practice note. Was there another long ransom note? We'll never know.

Vargas: (VO) Thomas says tear comparisons show that the ransom note came from pages 27, 28 and 29 of Patsy's note pad. Secret Service tests show the ransom note was written with a felt-tip pen found here, under the kitchen phone Patsy Ramsey used to call 911.

Vargas: Were John and Patsy Ramsey's fingerprint on the ransom note?

Thomas: No.

Vargas: No?

Thomas: No.

Vargas: (VO) But if they found the note and picked it up, Thomas asks why their fingerprints were not on it. Did they say whether or not they had picked it up to read it?

Thomas: I tried to pin Patsy Ramsey down at the time of our first interview with them. Did you grab the note? Did you pick up the note? Did you clutch it in your hand and read it and run upstairs with it? Who moved it to the hardwood floor? And I couldn't get an answer to that. She didn't recall.

Vargas: Is it possible that the parents could have handled the note and not left their fingerprints? Or that the paper for some reason didn't retain that kind of print?

Thomas: Certainly. But then I think the argument can be made, then when the sergeant touched the same pad, he left a fingerprint on it. When the CBI examiner touched the same pad, he left a fingerprint on it. Patsy had left previous fingerprints on that pad, five that we identified. So that remains one of the mysteries in this case. How come there's no identifiable fingerprints on this thing if one or both parents handled and grasped it that morning?

Vargas: Do you find that suspicious?

Thomas: Well, suspicious. It's just a big question mark that we'll — we'll never have an answer to, absent somebody confessing in this case.

Vargas: (VO) Thomas says other parts of Patsy Ramsey's story strike him as odd. For example, Patsy says she woke up that morning at 5:30, skipped a shower because she says it was broken, and dressed in the same clothing she wore the night before. In her book she writes, "I think to myself just put my clothes on. And, of course, my makeup. I remember my mother's words. Never leave the house without your makeup." Thomas thought it was strange that Patsy would overlook a closet full of clean clothes and apply all her makeup before heading down the back staircase to make coffee and find the ransom note.

Thomas: And indeed, under questioning Patsy Ramsey admitted she was in the same clothes the night before and that morning.