(Court TV) -- When Clara Harris drove to a Houston-area Hilton, she may have simply wanted to confront her husband about his adultery. But the confrontation turned violent. In the hotel parking lot, Harris slammed her Mercedes into David Harris three times and then left it parked on his lifeless body.

For Harris, a 44-year-old dentist, the naked truth may have been too much to bear: The day before her July 24 rampage she had hired a private investigator to uncover her husband's infidelity. She found him at the hotel with Gail Bridges, a secretary who had once worked at his orthodontic clinic.

The vigilant private investigator, who was situated in the parking lot with a video camera, filmed the fatal rampage.

"Our investigator had the camera focused on the subject until the very end," said Bobbi Bacha, who runs Blue Moon Private Investigators.

Catching a killing on film isn't de rigueur for a private investigator, but as PIs themselves are the first to admit the practice of vetting adultery allegations often results in violence.

"It's a tricky situation and there are a lot of emotions involved," said Bobby Newman of the Texas firm ACTA Investigatons, Inc.

There are no national statistics kept on crimes of passion in which investigators played a role, but most seasoned investigators have anecdotes and newspaper clippings at the ready when asked whether they, too, were involved in cases that went wrong.

In 30 years in the business, Newman says he has seen a number of adultery investigations turn dangerous. In 1990, he was retained by Arlene Rogan to trail her husband, whom she suspected of having an affair. He was. Distraught over Newman's reports, the wealthy socialite ultimately killed her husband and then committed suicide aboard their yacht. More recently, Newman trailed the husband of Lynn Ryan Kilroy, who was convicted this year of soliciting his murder.

For many firms, trailing potential adulterers is a necessary evil. The PI business bills by the hour, and adultery can be a cash cow compared to duties such as serving subpoenas or other court orders (Newman was retained by David Harris' daughter to serve a motion to freeze the couple's assets last week) and conducting interviews with witnesses, says Newman.

"I laugh when I hear about these firms saying they don't take divorce [cases]," said the PI. "It's just more business for me."

Why spy?

Business keeps rolling in for PIs only because couples keep cheating, says Ken Raggio, a lawyer with the Dallas firm Raggio & Raggio and a former chair of the American Bar Association's family law group. Couples considering divorce often have an incentive to catch philandering spouses with private investigators, says the lawyer, and hard evidence of adultery can open up the coffers.

In Texas and most other states, a so-called "fault" divorce on grounds of cruel treatment, abandonment, adultery and even addiction can give a spouse a chance at more than the normal 50-50 split.

Fault divorces were the norm three decades ago before the introduction of the amicable "no-fault" clause, now on the books in every state except Illinois and South Dakota. Then, private investigators were integral to the oft-staged adultery investigations needed to justify marital splits.

But today, says Raggio, those investigations can mean the difference between getting the house and getting an outdated set of encyclopedias. "You never know what you're going to need," said the lawyer. "You hope everything is going to work out, but sometimes ... if you have bad facts on the other side it persuades people that they don't want to play hardball."

Another motive, says the lawyer, is the "undying quest for the truth."

"If somebody feels that they have been living an ethical life, and they find out their spouse is leading a double life, they get pissed," said Raggio. "And sometimes when you find out the truth, you get angry."

And the truth isn't often pretty. "The majority of times people come to an investigator it's probably true," said David Kale, a California investigator who has worked on more than 15,000 cases since 1965. Emotional reactions happen regularly, says the investigator. "We always talk to the client later because you don't want that to occur," he says. "You don't want somebody to run off with a gun."

Keeping a safe distance

One precaution firms take to make sure clients don't have violent reactions is to keep them away from the scene of the investigation. "It says in our agreement that if our client is on the scene in any fashion that we will pull out of the case and they will lose any refunds," said Bacha.

Harris' rampage clearly illustrates the reasoning behind the rule. The successful dentist may have simply intended to catch her husband in the act. She had hired Blue Moon only the day before, after her husband reportedly confessed the affair and even revealed the location of the illicit trysts.

But soon after Harris arrived at the Hilton, with her husband's 16-year-old daughter from a previous marriage in tow, she became enraged. Harris demanded that a hotel employee summon the adulterous couple to the lobby.

When they arrived, a scuffle broke out between the two women, and Bridges lost her blouse. Hotel security guards intervened, and the combatants retreated to the parking lot.

David Harris was heading for his Lincoln Navigator when his wife first hit him with her Mercedes. The blow threw him into the air, and she hit him again before he landed. With her stepdaughter screaming and trying to exit the car, Clara Harris spun around to crush and recrush his body.

Not every cuckolded spouse is liable to snap like Harris, and that's why Kale says his business tries to choose clients who don't appear prone to knee-jerk reactions. Kale's first meeting with a client is generally something of a psychological evaluation.

Whether Harris actually snapped is a matter that will be left to the courts to decide. Out of jail on $30,000 bail, she told reporters after the rampage that it was an accident, and her attorney indicated last week that she plans to plead not guilty. In the interim, her conduct on July 24 was enough to convince one judge to issue an order of protection keeping Harris away from her husband's alleged mistress.