Aurora massacre suspect James Holmes is likely to be interviewed under the influence of a ‘truth serum’ if he enters a not guilty by reason of insanity plea over the killings.

The development came to light after Judge William Blair Sylvester ruled that a ‘narcoanalytic interview’ could legally be conducted in order to determine if Holmes was insane at the time of the shootings.

Defence lawyers have previously indicated that the former neuroscience PHD student is likely to enter a plea of not guilty by reason of insanity later today, so the likelihood of a full mental evaluation is high.

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If that evaluation takes place as expected, Judge Sylvester’s ‘truth serum’ ruling will almost certainly be acted upon, as will a second decision for Holmes to sit a lie detector polygraph test.

Holmes faces 166 different counts relating to last year’s shootings, which took place during a midnight showing of the Batman film The Dark Knight Rises.

Among those 166 counts are 12 murder charges and 70 wounding charges.

Holmes will enter his plea later today, with prosecutors then given 60 days to announce whether they will seek the death penalty.

If the death penalty is sought following a plea of not guilty by reason of insanity, legal complications are likely to develop - centred on the fact that the plea will require the defence to submit incriminating evidence to prove Holmes’ insanity.

That evidence is likely to be used against Holmes later in the trial, a complication that may breach his constitutional right not to incriminate himself.

Sky News reported Craig Silverman, a former deputy district attorney for Denver, as saying: “Was he suffering from a mental disease or defect that rendered him incapable of distinguishing right and wrong?”

Mr Silverman added: ”If the defence had to prove that, that would be one thing, but here in Colorado the prosecution has to disprove it. To prove sanity beyond a reasonable doubt with James Holmes could be tough.“

He went on to day that he expected the case to take a number of twists and turns, adding: ”I have been predicting delay after delay.”

“Delay is the natural ally of any death penalty defendant. The longer the case drags out the longer the client is assured to stay alive,” he said.