When Elisabeth Fritzl spoke to police at the end of her 24-year-captivity ordeal at the hands of her father, she shocked officers with her words.

"No one will believe me anyway," Elisabeth told Willibald Reitner, who in April 2008 was the head of criminal investigation in Amstetten, the Austrian city where Fritzl kept his daughter locked away for more than two decades.

He locked her away in a dungeon under his house when she was 18-years-old. Fritzl told his wife Elisabeth had run away with a cult. Over her years of being his prisoner, Elisabeth was raped thousands of times and had several children.

Her horrifying ordeal shocked the world - and now details have emerged of the tense few hours after she was first interviewed by police.

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Reitner tells the documentary, Fritzl: What Happened Next, that goes to air on UK television's Channel 5 on Thursday night (local time), that he and his investigators had to think of another way of getting her to talk.

"After that she said, 'If I tell this exactly how it all happened, no one will ever believe me'.

One of the conditions she had for co-operating with police were that she never had to see her father, who she referred to as "him".

"And she was really afraid that no one would believe her, and that everyone would only believe the father," reported The Mirror.

The abuse only ended when one of their children was taken to hospital and Fritzl allowed Elisabeth to go to the hospital. Police doubted the story they gave, that she had simply returned from living with a cult, and began to investigate.

The teenage daughter who went to hospital later died.

Elisabeth, now 51, is believed to live with her six children at a secret location in Austria.

She has never spoken publicly about her ordeal.

Reitner said he didn't regard Fritzl as human. But his daughter was another story.

They still keep in contact, exchanging messages at Christmas, but the most touching note came within months of the investigation finishing.

The letter reads: "If I may, thank you for helping us. You were there for us in a difficult situation. When we needed your support the most you took care of us, without great fuss.

"The experience of someone sticking by us, supporting us unconditionally, was like a gift to us. We are confident that we are not alone with people like you supporting us.

"We hope that you have someone supporting you, if you ever need help."

It affected him deeply, and he looks at the letter often.

Reitner still reads the letter regularly.

He told the Channel 5 documentary makers: "Naturally, I think of Elisabeth. I think of what she went through and above all I think about her thinking of us.

"Because policemen like me are always dealing with bad things and these words are the best things that can come back to us. No award would ever be as valuable as these words."

British reporter Mark Perry is also interviewed in the documentary.

He believed Fritzl was pure evil, reported The Sun.

"His eyes were cold and piercing. No remorse, no compassion, for anyone on earth but himself. No tears. Nothing.... [There] was no remorse. No compassion for his daughter, no compassion for his grandchildren who are of course, his own children."