THE man who murdered Irish student Karen Buckley will not be re-tried for sexual assault, with the Lord Advocate stating there is currently "no new evidence" to prosecute.

Alexander Pacteau was arrested as a 17-year-old following allegations he had tried to rape a woman in Glasgow four years ago.

He stood trial at the High Court in Paisley in 2013 but was found not guilty by majority and released.

In April this year, Pacteau attacked and killed Miss Buckley, 24, and tried to cover up the crime by hiding her body in a barrel of caustic acid.

A former family friend said last night the killer was a "troubled" child from the age of only four or five.

The 47-year-old, who worked with Pacteau's father Guillaume, said: "I remember him as a little boy, but he turned into a monster."

"I've never met a more obnoxious child. His bad behaviour was non-stop and he was constantly attention seeking.

"I've never seen a little boy like him and it wasn't something he grew into - he was born that way.

"But I could never have imagined this. I was absolutely sick when I found out what he had done."

After confessing to murder earlier this month, it was revealed he had earlier been acquitted of the sexual assault, leading campaigners to call for a fresh prosecution under double jeopardy laws.

The woman at the centre of the case has said her world fell apart after the verdict. She said she always knew Pacteau would strike again and even wrote to then Justice Secretary Kenny MacAskill warning authorities about him.

Clinical psychologist Dr Mairead Tagg told a Sunday newspaper: "It seems this woman did not receive justice. He should be re-tried if the evidence is there."

However, the Crown Office say they have already considered the use of double jeopardy but found there is no evidence which would warrant fresh proceedings.

A spokesman said: "As part of the investigation into the murder of Karen Buckley, the Lord Advocate gave consideration to an application under the Double Jeopardy (Scotland) Act 2011 in relation to his acquittal at Paisley High Court in 2013 for attempted rape.

"Applying the statutory test for double jeopardy applications, the Lord Advocate determined that there was no new evidence which would meet the test. As a result, the Lord Advocate decided that no application would be made to the court."

Applications under the Act can only be made where the acquittal was in relation to a High Court prosecution and one of three vital criteria must be satisfied in order for them to be successful.

One way is for prosecutors to present evidence that the accused has since confessed to the crime, or had done so before the trial but that the Crown could not reasonably have known during the first trial.

Alternatively, "tainted acquittals" can also prompt retrials if there is evidence of intimidation or interference with the jury at the original trial.

With regards to new evidence, prosecutors must present such material and show why it was not capable of being presented at the original trial.

This would likely include either an entirely new witness, or new evidence uncovered as a result of scientific breakthroughs.

Finally, the new evidence must be of such significance that had the original jury heard the material they would have been "bound to convict".

The Crown Office spokesman added: "Pacteau's conviction of the murder of Karen Buckley would not amount to new evidence for the purpose of Double Jeopardy legislation."

Double jeopardy was brought in 2011 following the collapse of the case against Angus Sinclair in the World's End murder trial.

He was re-tried last year and found guilty after new DNA evidence was presented to the jury.

However, the use of double jeopardy is likely to feature sparingly in prosecutions, such is the restrictive nature of the legislation.

Niall McCluskey, an experienced advocate in criminal law and a member of the Justice Scottish Advisory Group, said: "The circumstances in which double jeopardy can be invoked will be few and far between.

"There has to compelling new evidence discovered which was not capable of being presented at the first trial. It also has to be shown why this evidence was not presented at time."

He added: "It is obviously a terrible crime [Pacteau] has been convicted of and it does create concern when we find out that he was tried and acquitted of something not too dissimilar before.

"Some people may feel there is a moral imperative for him to be tried again but that does not meet the test in a legal sense."

Meanwhile, Police Scotland say they will "leave no stone unturned" if faced with a report of any other potential victims of Pacteau.

Detective Superintendent Jim Kerr has urged any other potential victims to contact police.

Pacteau is due to be sentenced at the High Court on September 8.