A law enforcement expert who specialises in detecting lies and deception has pinpointed an "area of concern" in a key witness statement in the Madeleine McCann case. 

Mark McClish, a former US Marshall and Secret Service agent, has analysed a statement given to police in 2008 by Gerry McCann's brother-in-law, almost one year after Maddie vanished. 

Parts of that statement which relate to blood and a strange odour in the back of a rental car should be explored further, McClish said.

In the aftermath of Maddie's disappearance Alexander 'Sandy' Cameron flew into Portugal from England with his wife Trish, and spent nearly three months with Kate and Gerry. 

During that time Sandy was listed as a registered driver for a silver Renault Scenic that had been rented by the McCanns. 

In late July of 2007, that vehicle became a lightning rod of interest to both Portugal's detectives and the huge media pack that had descended on the small town of Praia da Luz. 

As the hunt for the missing four-year-old intensified, a pair of British police dogs, one a specialist in sniffing out human cadaverine, the other a human blood canine, were brought to Portugal. 

The dogs were filmed alerting inside the McCann holiday apartment, next to a shelved wardrobe in Kate and Gerry's bedroom and behind a couch in the living room.

Later, both dogs again registered hits on the Renault Scenic which had been hired by the McCanns 25 days after Madeleine disappeared. 

Swabs were taken from the boot of the car and sent for forensics, revealing 15 of the 19 markers matched Madeleine's DNA. 

The report from British forensic scientist John Lowe stated the results were "too complex for meaningful inclusion and interpretation". 

In an attempt to try and make sense of the forensic results, Portuguese investigators requested Sandy Cameron make a written statement, which included answers to several questions about the Renault car. 

Cameron wrote how he would sometimes use the car to take black garbage bags from the villa where he and the McCanns were staying to the local dump. 

He also explained how blood from meat, fish and shrimps had leaked from shopping bags into the boot of the car, which was where human blood dog Keela alerted.

 McClish, who now trains police and military interrogators in the art of statement analysis, said Cameron's overall statement appeared truthful but that the part about leaked blood warranted further investigation.  

It was an "area of concern", according to McClish.  

Like cadaver dogs, statement analysis is not evidence admissible in court. Law enforcement agencies use these tools to assist investigations and zero in on potential line of inquiry.  

McClish told Nine.com.au that careful examination of words, phrases and language could help ascertain deception or truth.
 

"A truthful person's language will usually remain consistent.  

"Deceptive people will sometimes use synonyms because they are making up a portion of their story they cannot relate to and consequently do not always follow their personal dictionary."

Throughout his statement, Cameron used the word "vehicle" to describe the Renault until he reaches the point where he talks about the odour. Then the Renault becomes a "car".  

While recounting the blood and odour, Cameron also made a change in language between past and present tense.  

"The use of present tense is an indication this part of the story may not be coming from memory," McClish told Nine.com.au

The former cop said every small word in a statement has a meaning and justification.  

He added that, if he were an investigator, he would be inclined to ask Cameron more about the odour and the trips to the supermarket.

McClish noted how Cameron, despite have a much closer personal relationship to Gerry, ordered the pair as "Kate and Gerry" significantly more often throughout the statement.  

"It may be that he feels that Kate needs more support than Gerry, which caused him to mention her name first," McClish theorised.  

The McCanns have resolutely denied any involvement in the disappearance of their daughter.

READ: The full police statement of Alexander Cameron

READ: The official Portuguese police files on Madeleine McCann