Forensic linguists have put David Bain's 111 call under the microscope and showed that when it came to the alleged utterance "I shot the prick", it was more in the mind than on the tape.

And crucially, it seems police officers were more likely to hear the disputed phrase than others.

Bain's 1994 emergency call became a bone of contention in his 2008 retrial for the murder of his parents, two sisters and brother at the family's home in Dunedin, when he was found not guilty. A police officer digitising the tape thought he heard the previously unheard statement "I shot the prick". Though that part of the tape was successfully challenged by the defence and not played to the jury, it generated sensational headlines when media obtained the recording after Bain was found not guilty.

Australian expert in forensic phonetics, Helen Fraser, has studied the tape and played the disputed section to 190 people from Australia, New Zealand, the UK and the US.

Of those, only four people heard the breathy snippet as "I shot the prick" without being told what to listen for.

Fraser's interest was piqued when it was discovered that two of those four "unprimed listeners" were police officers, both were from Australia and had no prior knowledge of the case.

The 190 participants were then divided into two groups, with half being told the audio was thought to contain "I shot the prick" and half being told it was "He shot them all." Once the "I shot the prick" group were "primed" with the phrase in their minds, one-third of them said they could hear it being said.

The group remained high in their conviction of hearing it even after a phonetics "expert" refuted the evidence. Even at the end of the experiment when they were told it was generally accepted by experts that the tape did not contain those words and Bain was found not guilty, about half of the group remained convinced they heard the phrase.

Fraser said six of the participants were police officers, all Australian, and with no knowledge of the case. The officers were significantly more suspicious of Bain on the first listen and significantly more in the "he's guilty" camp than average at the end of the experiment.

Fraser said the research showed that letting police officers verify their own transcripts was "from a linguistic point of view ... madness".

For the record, Fraser said the tape had now been extensively examined and experts agreed that it did not say "I shot the prick".

There were two camps among the experts – either it was something starting with "I can't", possibly "I can't breathe", or it was just unintelligible noise.

Listen to the call and experience the test at: http://helenfraser.com.au/forensic/index.htm#transcription