Greek police are attempting to establish if adulterated alcohol contributed to the deaths of 18-year-old Irish students Andrew O'Donnell and Max Wall on the Greek island of Ios last weekend.

O'Donnell and Wall, who both finished their Leaving Cert exams at St. Michael's College in Dublin last month, died in separate incidents in Ios over the weekend. 

O'Donnell was found in a rocky area near the town of Chora on Sunday, July 2 after being missing for over 24 hours and is believed to have suffered a tragic accident, while Wall fell ill and collapsed while waiting for a ferry to leave the island later on Sunday.

Samples from alcoholic drinks sold in all bars and clubs on the island have been sent to a chemical laboratory on the island of Syros for testing. However, Greek prosecutor Ekaterini Lyra said the tests were a standard investigation procedure, adding that there was no evidence to suggest that adulterated alcohol contributed to either death. 

Lyra, the district prosecutor on the nearby island of Naxos, told the Irish Times that both post-mortems were completed yesterday but said an analysis of toxicological samples taken during the autopsy was pending. 

Brigadier Thanos Loukas, head of the regional police department, told the Irish Mirror that there is "no reason" to suggest that any criminality was involved in either death. 

It is understood that O'Donnell and Wall's bodies will be returned to their families on Thursday, July 6 to begin the repatriation process. 

Two officers from An Garda Síochána have traveled to Greece to assist in the investigation and will also assist in the repatriation process. 

Earlier in the week, up to 500 Irish people lined the route between a health center in Ios and the island's port as the two bodies were taken to Athens for post-mortem investigations. 

Bars and clubs on the island have remained shut since the two deaths as a mark of respect, while mourners have placed flowers under the cactus tree where O'Donnell's body was found and down the side street near the port where Wall collapsed. 

Several people reported seeing Wall passing out fliers looking for his friend after O'Donnell disappeared. He reportedly collapsed shortly after hearing that O'Donnell's body was discovered. It is understood that Wall suffered from a heart condition in the past. 

Meanwhile, a service has been held in memory of O'Donnell and Wall at their school in Dublin. 

St. Michael's principal Tim Kelleher told the service that up to 60 students from the school wished to remain on the island out of "solidarity" after news broke of the two deaths. 

"It’s a mark of those men and mark of all the boys that were there searching – some of whom who are here today – who went out searching for Andrew," Kelleher told the service. 

"We’re so thankful to you. Our hearts go out to you and the pain you’re suffering."