Gardaí investigating the murder-suicide in Bray, Co Wicklow, believe Shane Clancy may have been taking too many of the anti-depressant tablets that had been prescribed for him just a week before he took his own life.
Detectives do not believe the 22-year-old Trinity College student was under the influence of any illegal substance when he murdered Sebastian Creane and stabbed and seriously injured his ex-girlfriend, Jennifer Hannigan, and Creane's brother Dylan.
The main line of inquiry being pursued is that Clancy may have been taking more than the recommended dose. Gardaí believe that when he found out that Creane and Hannigan were dating he "snapped and lost control" and bought knives before attacking his three victims. Toxicology reports are due back within the next few weeks.
It is understood Clancy had complained to friends in the days before the incident that he did not like the way his medication made him feel.
There has long been a debate within the medical community about the potential side effects of anti-depressants. Some experts have claimed that certain medication can cause aggression in some users while the drug companies say there is no clinical evidence for this.
Garda sources say they have no evidence that Clancy was stalking or harassing his ex-girlfriend and that it was actually him who ended the relationship five months ago.
He later had a change of heart but Hannigan did not want to get back together with him. Despite this, the pair were said to have been on relatively good terms and nobody to whom gardaí have spoken suggested Clancy had ever shown a propensity for violence.
Hannigan began going out with Seb Creane, her college classmate, three weeks ago. She has given gardaí a detailed picture of what happened in the house in Cuala Grove in Bray after 4.30am last Sunday morning.
After buying a set of three knives in Dunnes Stores in Cornelscourt, Clancy drove to Creane's house and stabbed him in the heart before attacking Hannigan, leaving the blade of a knife lodged in her back. He then stabbed 28-year-old Dylan Creane eight times, puncturing his lung.
When gardaí arrived at the scene Clancy was nowhere to be seen and a manhunt was launched. He had committed suicide in the back garden and his body was discovered by the Garda Technical Bureau on Sunday morning.
Shane Clancy and Seb Creane had been socialising together in Dalkey before the murder and Clancy had given him and two other friends a lift home.
The two friends have given statements to gardaí. They said Clancy and Creane were getting along well and there was no indication that Clancy bore any ill-will towards him.