Watch the full interview with Madeleine McCann's parents, four years after her disappearance, on Piers Morgan Tonight, 9 p.m. ET.
(CNN) -- Four years after the disappearance of Madeleine McCann, a British toddler who went missing in Portugal, her parents have written a book detailing their account of that horrible night.
"I suppose there's always been the urge to -- to get the truth out there," Kate McCann, the missing girl's mother, told CNN's Piers Morgan in an interview. "Obviously, when there's been so many stories written. But we've always had things to weigh up. And ultimately, Madeleine's fund was running out, and I knew that we'd need to raise money, really, to continue to search."
She says profits from the book sales will be directed toward furthering the investigation into the disappearance of their daughter in Portugal's Algarve region in 2007.
"There's just always something inside, and the pain is never too far away from the surface," said Gerry McCann, the girl's father.
News of the missing wide-eyed girl grabbed headlines across the globe and moved celebrities such as soccer star David Beckham to make public pleas for her safe return.
"I don't think it consumes every minute as it did before," Kate McCann explained. "But certainly, you know, Madeleine's absence is there constantly."
She added that she's "now reached that point where I will allow myself to take time out, and just relax a little, enjoy something." But the absence of her daughter is "still tangible."
The pair says they are hopeful that the book, titled "Madeleine," will prompt others to come forward with information about the girl's disappearance.
"There is a chance she may not be alive," said Kate McCann. "But what we do know is there's a very good chance that she is alive."
Madeleine was nearly 4 years old when she vanished from her family's holiday apartment in Praia da Luz as her parents dined in a nearby tapas restaurant with friends.
Despite a huge police investigation and massive coverage in the Portuguese and British media, she has not been found.
At one point, authorities in Portugal had named the girl's parents as suspects, as well as a British man living in Portugal, Robert Murat.
But a spokesman for the Portuguese prosecutor's office said in July 2008 that authorities found no evidence of involvement by any of the three and they were no longer considered suspects.
Kate McCann said earlier that she thought being named as a suspect had a devastating effect on the case.
Portuguese investigators closed the case in July 2008.
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