Experts investigating last October's deadly inferno inside the Gotthard Tunnel have released their final report into the incident.
Eleven people lost their lives in the tunnel when two trucks collided near the southern entrance, sparking a devastating fire.
A team of accident investigators spent months combing through the wreckage of the burnt-out trucks in the search for clues as to how the fire started.
Presenting the results of their six-month-long investigation into the fire at a press conference on Thursday, experts said they had been unable to find any technical defects on either vehicle.
Investigators said a Turkish truck driver, Seyfi Aslan, slammed into a tunnel wall before skidding into the path of oncoming traffic.
The report concludes that an Italian truck driver, Bruno Saba, swerved to avoid a head-on collision but was unable to avoid smashing into the side of the other vehicle.
The fire broke out when one of the truck's fuel tanks ruptured, causing an explosion inside the tunnel.
Experts believe most of the 11 people killed in the ensuing blaze died of asphyxiation from the toxic gas which filled the tunnel.
The tunnel was shut down by authorities immediately after the fire and was only reopened shortly before the Christmas holiday.
The closure of the Gotthard - Europe's main north-south transalpine axis - caused widespread congestion and long tailbacks at both ends of the tunnel.
The Gotthard, the main gateway through the Alps from Germany to Italy, is normally used by about 19,000 vehicles a day.
In the days and weeks following the fire, thousands of drivers were forced to use alternate routes through the Alps to complete their journeys, leading to long delays on the approach to Switzerland's San Bernardino tunnel.
The San Bernardino, which is not equipped to handle large numbers of heavy goods vehicles, became a flashpoint for accidents in the aftermath of the Gotthard fire.
In 1999, a similar fire inside the Mont Blanc tunnel which links France and Italy killed 39 people.
French authorities only gave the green light to the gradual resumption of traffic through the tunnel in March of this year.
swissinfo with agencies