Former New York City police officer Gilberto Valle, dubbed by local media as the

Former New York City police officer Gilberto Valle, dubbed by local media as the "Cannibal Cop", listens as his wife Kathleen Mangan-Valle testifies in this courtroom sketch. Photo: Reuters

WARNING: GRAPHIC CONTENT

The wife of a New York policeman accused of plotting to kidnap and cannibalise women has sobbed as she testified about discovering her husband had gone online to discuss torturing and killing her.

Kathleen Mangan-Valle was the star witness for the prosecution in a trial that opened with her telling the jury that Gilberto Valle had a desire to "cannibalise human flesh."

Mangan-Valle said she first caught her husband, 28, and who has pleaded not guilty to a kidnapping conspiracy, visiting websites dedicated to violent sexual fetishes while they were raising their young daughter.

Repeatedly crying in the crammed Manhattan federal courtroom, Mangan-Valle described her decision to flee the house with the baby after finding horrific pictures on a computer from one of Valle's late-night internet sessions.

"The girl on the front page was dead," she said. Another website her husband had visited featured "pictures of feet that were not attached to bodies".

On closer inspection, Mangan-Valle said, she realised her husband had exchanged "thousands of emails" with other death porn enthusiasts and that he'd shared photographs of people they knew.

"Suddenly I was staring at pictures of me, at pictures of friends," she said.

In one plan discussed online, "I was going to be tied up by my feet and my wrists and my throat slit".

Overcome, Mangan-Valle had to briefly leave the courtroom. Her husband sat at the defendant's table with his head in his hands.

But Valle's attorney, Julia Gatto, countered that the six-year veteran of the New York Police Department had only been fantasising the whole time, and said her client was a good family man, despite a "very bizarre" sexual fantasy.

The intense testimony, during which Mangan-Valle recounted turning her husband over to the FBI, hit a dramatic pitch when Gatto showed the court photos of the estranged couple in happier times.

One showed Valle in police uniform cradling their newborn baby in hospital, illustrating Gatto's argument that her client was not a dangerous husband.

Seeing the pictures, Mangan-Valle and her husband both broke down, the accused man slumping in tears at the table where he sat and the woman accusing him crying so hard on the witness stand that she could not talk.

Valle is accused of conspiracy to kidnap and illegally accessing law enforcement databases for detailed research on potential victims.

The allegations are so gruesome that the court had to screen potential jurors for their views on extreme sexual fantasies, even showing them a photograph of a woman trussed up in a cooking pan, with an apple stuffed in her mouth.

Valle has pleaded not guilty but faces a maximum sentence of life in prison if convicted on the kidnapping conspiracy charge.

The officer, who had been aiming for promotion to sergeant, is in custody in Manhattan at the Metropolitan Correctional Centre's Special Housing Unit for high-security prisoners.