AS DETECTIVES questioned Wayne O’Donoghue about semen found on Robert Holohan’s left hand, the 20-year-old engineering student didn’t flinch. A video camera recorded his every word. The interview, according to gardai, was “tough going”, but O’Donoghue held his nerve.

“There was no good-cop, bad-cop routine,” said a senior officer. “It was bad cop all the way. But O’Donoghue was a cool customer and he didn’t sway.”

What would he say, gardai asked O’Donoghue, if it turned out that his semen was on the dead 11-year-old boy’s body? O’Donoghue replied confidently that he was sure that would not be found. He volunteered a sample of his DNA and later suggested to gardai that Robert must have “picked it (the traces of semen) up from a towel”. A towel that O’Donoghue used to dry Holohan’s dead body in his bathroom.

The interview took place on January 16, 2005, the day after Robert was buried. O’Donoghue had just admitted to his father, and to gardai, that he had accidentally killed his next-door neighbour. He strenuously denied any suggestions of a sexual motive for the killing. He was charged with manslaughter.

In the days following the discovery of Robert’s body, a post-mortem examination carried out by Marie Cassidy, the state pathologist, concluded that there was no evidence that the schoolboy had been sexually abused. But Cassidy had found semen on Robert’s palm.

The sample was sent by gardai to Yorkshire for analysis by John Whitaker at the Forensics Science Service laboratory at Wetherby. The sample was small, but Whitaker’s team tested it using a new DNA-testing technique, low copy number, which allows matches to be found from very few sample cells. It was compared with O’Donoghue’s DNA.

Whitaker’s report landed on the desk of Liam Hayes, the superintendent leading the investigation, last March. Whitaker concluded the likelihood of the semen sample coming from anybody but O’Donoghue was one in 70 million. On the basis of Whitaker’s report, James Hamilton, the DPP, upgraded the charge from manslaughter to murder on April 21.

Detectives investigating the killing were relieved. The force’s record in headline murder cases is poor. The public interest and media scrutiny surrounding Robert’s death compounded the need to secure a murder conviction. Whitaker’s report seemed to be the breakthrough gardai needed. A trial date was set for last October.

But last July the prosecution case was dealt a mortal blow when Whitaker, their star witness, cast doubt on his initial findings. A second semen sample was found on a mat in O’Donoghue’s bathroom, where he had taken Robert’s dead body. O’Donoghue said he splashed water on the body in an attempt to revive the boy.

Forensic reports on this second sample found it to be similar to the semen in Robert’s hand, but the tests could not confirm irrefutably that it was O’Donoghue’s. Whitaker expressed doubt about his first analysis, to the point that he now refused to give a statistical likelihood of the first sample being O’Donoghue’s.

“It was like a Scud missile had just exploded,” said one senior officer. “It created untold difficulties. The murder charge had already been entered by the DPP and Whitaker’s first report gave credence to our belief that Robert had been murdered and the motive for the crime was sexual. But the later reports blew that apart and we were gutted.”

Despite the setback, the DPP decided to press ahead with the murder charge, but upon receipt of Whitaker’s reports, Frank Buttimer, O’Donoghue’s solicitor, asked another independent expert to review the forensic evidence. Buttimer’s expert poured scorn on the reliability of the forensic evidence. “This was the only battleground in the case, and it was settled before the trial even began,” said Buttimer.

Last November, two weeks before the trial started, the DPP met senior gardai and informed them the semen evidence would be excluded. It would be unsafe to introduce evidence about the sample as it would be prejudicial against O’Donoghue and could scupper the state’s case, said Hamilton. But he again refused to withdraw the murder charge.

Times Recommends

• 'Radio Rancid' is scourge of Obama
• French gulp as global warming alters wines
• Colton, the barefoot boy bandit, outfoxes sheriffs