It's often said that the only certainties in life are death and taxes. But you can add "rehashing of the O.J. Simpson case" to that list -- at least for the last 18 years.

So it should come as no surprise that a new book has been published about the 1994 murders of Simpson's ex-wife, Nicole Brown Simpson, and her friend, Ron Goldman.

In 1995, a California jury acquitted O.J. Simpson of the killings. A civil lawsuit, later filed by the victims' families, resulted in a 1997 judgment finding Simpson liable for the deaths and ordering him to pay $33.5 million in damages.

The latest installment in the Simpson library is not another "If I Did It," in which the former gridiron great speculated on how he might have killed his former wife. Instead, the new book points the finger of guilt away from Simpson and lays the blame on his son, Jason Simpson.

"Everything we have in the book is documented. It is not theory or hypothesis. It is fact," renowned private investigator William C. Dear told The Huffington Post about his book, "O.J. Is Innocent and I Can Prove It."

Dear's 576-page "true account," according to Amazon.com, hit the shelves today, retailing at $18 for the hardcover edition.

In the investigation into the murders of Brown and Goldman, Jason Simpson was never considered a suspect or a person of interest. The 41-year-old lives in Miami, where he reportedly works as a chef. HuffPost was unable to reach Simpson for comment Monday because his phone had been disconnected.

But Dear said he has spent nearly two decades looking into the case and assembled a mountain of circumstantial evidence, which, he said, suggests that O.J. Simpson had nothing to do with the murders of Brown and Goldman.

"I flew out two weeks after the murders," he said. "I climbed over the back gate and walked the walkway to the front door, and that's when I realized O.J. could not have done it. But he was there. He was either there at the time or there afterwards [and] became part of the crime."

In his book, Dear claims that he has the knife used in the murders, along with photos and other evidence that suggest the true killer was Jason Simpson, O.J.'s son with his first wife.

"When I tell you we have the weapon -- we found the weapon in Jason's storage facility that he failed to make payments on. We know he carried it -- his initials were carved in the leather sheath," Dear said.

"We have emails from his former roommates that were in college with him. We have our suspect's diaries. We have his forged time card, and we have the vehicle he was driving on the night of the murders," said Dear.

The private investigator also claims to have photos of Jason Simpson wearing the knit cap that was found at the murder scene.

But why? Why would Jason Simpson kill Brown and Goldman?

During O.J. Simpson's trial, prosecutors alleged that the defendant was obsessed with his ex-wife, that he was prone to jealous rages and that he would stalk her.

Dear contends that Jason Simpson has his own demons and suffers from "intermittent rage disorder."

"Our suspect at the time was 5'11" and 235 pounds," Dear said. "He was 24 years old, and he was on probation for assaulting his previous employer with a knife. In addition to that, he's had three attempted suicides and has been in a psychiatric unit."

On the day of the murders -- June 12, 1994 -- O.J. Simpson and Nicole Brown attended a dance recital for their daughter. Dear alleges that Jason Simpson was working as a chef in a Beverly Hills restaurant and had put together a special meal for the family. Brown, however, did not attend.

"You're dealing with a young man who just weeks prior had checked into a hospital where he said he was out of his medication and was about to rage," Dear said. "I have no doubt he had no intention of killing her, but [he] confronted her and, as a result, something happened."

Dear said the diaries he obtained, which were allegedly written by Jason Simpson, refer to the young man's obsession with knives and the problems he was purportedly dealing with.

One entry allegedly reads, "It's the year of the knife for me. I cut away my problems with a knife. Anybody touches my friends -- I will kill them. I'm also tired of being Dr. Jekyll [and] Mr. Hyde."

O.J. Simpson was unavailable for comment at the Lovelock Correctional Center in Lovelock, Nev., where he is serving a 33-year prison sentence. In 2008, he was found guilty of armed robbery and kidnapping for taking sports memorabilia from a dealer at gunpoint.

While the book's bombshell claims have not been proved -- authorities in California have yet to comment on them -- Dear insisted he can back up every allegation.

"I have been inducted into the Police Officer Hall of Fame as a private investigator, so my credentials are not [that of] some idiot guy just throwing it out there. My reputation is important to me. I would not say any of this without a great deal of backup," Dear said.

Dear also contended that he has managed to convince others that his theory has merit.

"I recently did a speech in front of 533 law enforcement investigators and prosecutors," he said. "The first statement I made was 'How many of you believe O.J. was guilty?' and everyone raised their hand. When [my speech] was over, I asked the same thing and only three people voted guilty. So when you get law enforcement and all these people to take that position, that's a pretty strong position."

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