In the case of the McCanns, Keppel says that Portuguese police should have been more proactive in their treatment of the parents. "If the parents are claiming that the kid was abducted, they should have processed them right away for physical evidence -- scars, marks, blood stains on clothing."
Unfortunately, police in these circumstances are often governed by the emotion of the moment, says Keppel. "They veer between two extremes: If they're only treating them as suspects, that can be a problem. And if they only treat them as grieving parents, that can be a problem."
Michelle Oberman, a law professor at Santa Clara University of Law and the author of "Mothers Who Kill Their Children," was also surprised that the police didn't question the parents right away.
"It's the most natural place to start," she says. "The question is why didn't they do this three months ago. The numbers are pretty astonishing, that young children are at greatest risk of homicide by their parents."
One parent who's been in the hot seat, as a sympathetic figure and as a possible suspect, is Ed Smart, the father of Elizabeth Smart. His daughter was abducted at age 14 by a homeless handyman and his female companion and kept for nine months before being reunited with her family in 2003. During the ordeal, the police seized Ed Smart's computer and questioned him and his brothers about Elizabeth's disappearance.
Smart says that he believes in the McCanns and that he talks to Gerry McCann every week, offering him advice and support. "I have been in touch with Gerry ever since the end of July when he came to the U.S. and it was recommended that we talk because of the similarity of our cases," Smart tells ABCNEWS.com.
"In either case, I don't think that the police were trained to deal with the situation, and I don't think that it was done correctly to begin with," he says. Smart claims that the police leak information and innuendo to the media, are prone to tunnel vision and become fixated on a single suspect without following all the leads.
Smart questions some of the recent evidence in the McCann case, including microscopic traces of blood found on the walls of their apartment, saying he's talked to Gerry about it. "He said to me, 'I don't know who's been in the apartment that we've been in -- they might have cut themselves.'"
He also questioned the use of sniffer dogs in the case. "We had 10 dogs when Elizabeth was abducted and they looked at two or three areas of our backyard before losing the scent," he explains. "I don't have a lot of faith in the dogs because of what happened to us."
Smart believes in the innocence of the McCanns: "I do not believe for a minute that anyone who killed their daughter would be staying in Portugal for four months and go on international trips to publicize the loss of their daughter."
The couple also attracts the sympathy of Barry Cohen, the lawyer for Marlene and Steve Aisenberg, the couple once charged with lying about the 1997 disappearance of their infant daughter, Sabrina.
"Accidents happen and it's a tenable theory that the parents panicked and concocted this story about a kidnapping and went to dispose of the body," he explains. But Cohen questions the evidence: "How do you get blood in the rented car if the girl wasn't in that car?"