By ZOE BRENNAN

The telephone call came in the dead of the night. Told that a battered wife needed her help, Margo Bennett rushed to the Prince Of Peace United Methodist Church. Bursting through the door, she stopped dead in her tracks.

In the corner of the church vestry sat the minister, the Reverend Edwin Clever, a bag over his head, his hands cuffed and ankles shackled, and with explosives tied around his waist.

A man dressed in black and with a stocking obscuring his face pointed a gun at Margo's head. Recognising his voice, an ominous chill ran down her spine.

Patricia Cornwell at the British Book Awards, April 2008

She knew it was her estranged husband Gene, a former FBI agent - just like her.

Enraged by his wife's lesbian affair with millionaire crime writer Patricia Cornwell, Gene had determined to murder Margo, the mother of his two young daughters. Now he had sprung his trap.

Margo shouted at the Reverend: "Edwin, are you praying? Pray for me, too. I'm a little busy right now."

Then, as the bulky frame of her husband moved towards her, Margo's grip tightened on a can of pepper spray hidden in her handbag.

She sprayed him directly in the face before getting out her own gun and telling him: "You're not going to kill me, Gene. I am not going to let this happen."

Then she pulled the trigger. The bullet slammed into the door next to Gene, and as he tried to escape - the effects of the pepper spray still blinding him temporarily - she grabbed a phone and called the police.

They arrived within minutes, arrested Gene Bennett and released the Reverend from his explosives belt - which turned out to be a clever fake.

It was the final chapter in a most extraordinary story of greed, lust and violent revenge. Indeed, it had all the ingredients of one of Patricia Cornwell's bestselling books featuring pathologist Dr Kay Scarpetta, but with one crucial difference: it was all true.

Now, this story - which would result in awful misery for the characters involved and a humiliating ordeal for Cornwell herself - is being told in all its extraordinary detail for the first time in a book, Twisted Triangle, written by Caitlin Rother and John Hess.

So just how did the now 51-year-old celebrated American author come to tangle with Margo and her husband in this lurid affair?

In fact, it was Cornwell's desire to make her novels as realistic as possible which first brought her into contact with the pair through their work for the FBI.

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Margo had met Gene Bennett in 1982 in Atlanta, Georgia, as a rookie FBI agent. At 6ft 2in, Gene dwarfed the slender blonde Margo, who found herself charmed by his confident swagger.

A skilled undercover agent, he was at the time running Operation Nickelride, one of the FBI's most successful clandestine projects.

Posing as a cocaine-dealing diamond thief, he recovered $8.5million in stolen goods and contraband and brought in 240 criminals in one audacious swoop.

He rented out an Italian restaurant called Tony's and ferried in the underworld suspects by limousine to meet a fictitious Mafia boss.

When they arrived, they were arrested by a SWAT (Special Weapons And Tactics) team. The case was duly lauded as a spectacular coup.

For her part, Margo was affectionately known in police circles as the "Blonde Bitch" for her tenacity and drive.

By now an FBI golden couple, Gene and Margo married in 1984 and had their first child, Allison, two years later.

Soon, however, Margo became aware that her husband was involved in dodgy deals, including a jewellery business with one of his informants.

When the couple moved to Washington DC, he became involved in an illegal scam to claim relocation expenses for a house they had never lived in.

He coerced Margo into going along with the scheme, which netted him $17,873 from the FBI.

"I felt I had sold my soul to the devil and I couldn't get out of this," she said. Soon, he began to make fraudulent insurance claims, too.

In 1987 Gene was promoted to the Counter-Intelligence Squad, monitoring the KGB. He was increasingly unhappy at work, however, and in their home and sex life - pestering Margo to take part in a threesome.

She had been involved with a woman before meeting Gene, but kept this secret, saying: "Something was telling me: 'Don't expose that part of you to him.'"

Despite a growing gulf between them, the couple had another daughter, Lindsey, born in 1989. By now, Margo worked as an instructor at the FBI, teaching interrogation skills. She wanted to leave Gene, but was scared.

"I knew how he was when people turned against him," she says. Given what was to follow, this seems prophetic.

Enter Patricia Cornwell into this unhappy saga.

On June 17, 1991, a colleague of Margo's brought an upandcoming novelist friend to the office. She went by the nickname of Patsy, and wanted to gain further insights into the criminal mind.

Margo popped out to the nearest bookshop and bought Cornwell's first two forensic thrillers, Postmortem and Body Of Evidence, to get them signed by their author.

As they shook hands, Cornwell said to Margo: "Perhaps you can teach Scarpetta a thing or two." Margo replied: "I doubt it, but I look forward to it."

The following year, Cornwell visited again, and the pair chatted for hours. The talk was flirtatious, and Margo remembers that Cornwell kept swivelling around on her office chair - touching Margo's leg seductively with the toe of her shoe.

"She was very soft-spoken," says Margo. "She seemed to have a lot of hurt in her life. She wasn't clingy or needy, she was just very intense. She seemed to have a lot of depth in her, a lot of experience."

The pair talked several times a week on the telephone, and Cornwell bought her new friend expensive presents, including a Mont Blanc pen.

Soon, Margo invited Cornwell for dinner, during the course of which the novelist took her hostess's hand and began stroking it erotically.

"She was making love to my hand," says Margo. "It was like my whole system was in overdrive."

Attending an FBI seminar together the following month, Margo hugged Cornwell goodnight. "I can feel your heat," said the writer. "I can't believe how hot you are."

Margo began to kiss her. "It was a total sensual bath of feeling," she recalls. "It was so astounding, it took my breath away.

"Up to that point in my life, that was the most tender kiss I had ever had, and yet at the same time, it was the most ferocious in the intensity of it and what it was doing to the inside of me. I was mush. The hardest thing I had to do was let go of her and say: 'I have to go home.'"

The next day, Cornwell and Margo had a long heart-to-heart, during which Cornwell talked about her mother, who suffered from mental illness. She told how when she was young, her brother had kept a gun in their bedroom in case their mother came in.

Next, Cornwell asked her lover to her home. She led Margo directly into the bedroom and her king-sized bed.

"It was a very special and magical, unique moment," says Margo. "Not something I'd ever felt before."

Despite still being married to Gene, Margo began to accompany Cornwell to literary events, including a party for the release of another Scarpetta bestseller, All That Remains, after which they again went to bed.

The novelist told Margo that she "really had a thing for" Oscar-winning actress Jodie Foster, who played FBI agent Clarice Starling in Silence Of The Lambs. Later, Cornwell brought actress Demi Moore to visit the FBI office.

Meanwhile, Gene became ever more threatening. Eventually, Cornwell told Margo: "He scares me. I don't trust him, he's dangerous." She began backing away from the relationship.

Margo told Gene she wanted a divorce. She rented a new home to move into with her daughters and told her FBI bosses about Gene's house fraud scam.

Gene was arrested and charged, but four days before the trial in 1993 he attacked Margo, who was due to give evidence against him.

Slamming her against the wall of a garage, he shot her above the right eye with a Taser stun-gun, sending 50,000 volts through her skull. He fired a further four times, until she lay limp on the floor.

He told her his criminal friends - who were implicated in the fraud case - were holding their daughters, and Margo had to change her evidence.

The prosecution lawyers were stunned when she took to the stand and reversed her testimony. The judge granted a recess, and the lawyers persuaded Margo to tell the truth about why she had changed her story.

It turned out that, far from being held by hoodlums, her children were in the care of a babysitter hired by Gene. A new trial was scheduled.

But Margo was terrified that Gene would use her lesbian affair against her and she would lose custody of the girls, so she phoned Cornwell to break off their relationship for good.

Margo says: "It was a very realistic decision to make and, unfortunately, that's what we had to do."

Shortly afterwards, Gene received a one-year prison sentence for fraud.

Despite having turned in her husband, Margo was fired by the FBI.

In April 1995 Gene was released from prison - plotting revenge.

Posing as a private investigator, he hired a woman named Mary Ann Khalifeh and hatched an elaborate scheme reminiscent of his early undercover work, taking out a $250,000 accidental death policy on Margo's life.

At the same time, he set out in divorce papers how Margo had had a relationship with Cornwell, "secretly meeting for romantic candlelight dinners".

He embellished their affair, saying he had found their "lingerie, sex toys and lesbian pornographic material".

He asked for sole custody of his children, saying that he didn't want his daughters "raised by a lesbian".

Midway through the custody hearings in 1996 came the dramatic confrontation at the Prince Of Peace United Methodist Church.

After his arrest, Gene put on an elaborate pretence of insanity - which fell apart at his trial.

Among the items found around the church were a stethoscope, several syringes, saline solution, blindfolds and a tube of glue. Five further hoards of material would be found at other locations.

The "death packs" contained razor blades, lesbian pornography, a book titled A History Of Witchcraft, a gas mask, latex gloves, and explosives.

In one, something even more sinister was found: a strange foot-long contraption, wrapped with shiny black electrical tape.

It was a homemade bomb - all the more peculiar because it had a black vibrator attached to it. When detonated by bomb disposal experts, it was found to contain a deadly mix of fishhooks and nails.

A map of Patricia Cornwell's neighbourhood was also discovered.

At his trial seven months later, it would emerge that Gene had been planning to frame Margo for planting the bomb, making it appear as if she was having a lesbian affair with Khalifeh, his assistant.

He was planning to torture Margo by injecting her with a poisonous substance before murdering her.

Gene was sentenced to 23 years in jail for charges including attempted murder. Margo was awarded full custody of their daughters.

At the time of the trial, Cornwell said: "My personal life is not anybody else's business. I don't believe people should be defined by their sexuality. People can think what they want."

Later, she told a magazine: "It [the affair] was very brief in every way you can imagine."

Margo says of Cornwell's comments: "Did it hurt my feelings? Yes. Did I spend time dwelling on it? No."

In 1996 Cornwell telephoned Margo, asking for dates she needed for a lawsuit, and offered to help her write a book of her story. It was the last time they spoke.

Movie producers tried to buy Margo's story, but she wanted to rebuild her life instead. Aged 54, she is now in a new relationship with another woman.

Gene's appeal was rejected in March 1999. He is eligible for parole in 2016, when he will be 61. Margo believes that she is ready should he try to take final revenge when he is released.

She says: "God didn't let me survive the first two attempts to have me fail if he tries again."

• Adapted from Twisted Triangle by Caitlin Rother and John Hess (John Wiley, £14.99). To order a copy (p&p free), call 0845 606 4206.