Police have announced that Ylenia, a five-year-old girl from Appenzell whose body was found in September, was poisoned with toluene - a chemical used in solvents.
Her suspected killer, a "dangerous and calculating" 67-year-old Swiss man, had prepared the crime several weeks in advance, the investigating authorities said on Monday.
"The high dose of [toluene] led to her death. Ylenia's body did not reveal any signs of a sexual attack, but this doesn't exclude the fact that the motive of the abduction may have been sexual," the examining judge, Erich Feineis, told a news conference in the eastern city of St Gallen.
Her abductor had earlier used another solvent to render the young girl unconscious.
On the day she disappeared from Appenzell swimming pool - July 31 - the suspected kidnapper committed suicide at a wood near the village of Oberbüren after shooting at and wounding another man in the chest.
Despite a massive police search, Ylenia's body was not discovered until September 15 by a man walking in the same wood where the shooting took place.
All evidence points towards the involvement of the 67-year-old, the investigating authorities said on Monday.
"The fact that he had weapons with him and a spade indicated that he was planning a murder," Feineis said. "But it seems that he only decided to commit suicide after being surprised by a witness who he shot at."
Police believe Ylenia was abducted in a white van with Spanish licence plates. An analysis of DNA taken from the vehicle indicated that it had been used to transport the girl.
Her suspected kidnapper was a resident of eastern Switzerland before resettling with his wife in Spain in 1990.
Missing children and child murders are rare in Switzerland and the case of Ylenia made the front pages of national newspapers and magazines.
The case drew parallels with that of missing British girl Madeleine McCann.
In September, 100 Swiss personalities signed an open letter to the government demanding a faster system to track down abducted children.
It called for an "abduction alert" to be issued as soon as a child is reported missing. The signatories include champion skater Stéphane Lambiel, former ski champion Pirmin Zurbriggen and billionaire Ernesto Bertarelli, who runs the Alinghi yachting team.
In October the House of Representatives came out unanimously in favour of proposals demanding the introduction of an alert system using mobile phones and involvement by the public to find missing children.
The proposal, which still has to be debated by the Senate, also foresees the use of electronic media - including the internet - as well as signboards at stations, airports and border posts and along motorways.
The government agreed to take action, but pointed out that the country's 26 cantonal authorities are responsible for criminal proceedings in the matter of presumed abductions of children.
swissinfo with agencies
Ylenia from Appenzell in eastern Switzerland was last seen at a swimming pool on July 31, according to police, who have declined to release her surname in keeping with Swiss privacy laws.
That evening police found her backpack, cycling helmet and scooter beside a path in woods about 30 kilometres from the swimming pool.
On August 1, in the same forest, police found the body of a 67-year-old man who they said had shot himself in the head with a pistol.
A day earlier he had shot and injured a 46-year-old man. The younger man fled and was taken to hospital and later released.
Police said the dead man, and suspected kidnapper, was a Swiss native who had lived in Spain with his wife since 1990.
Ylenia's corpse was discovered by a passer-by on September 15.
It is still unknown exactly what the 67-year-old did with Ylenia, with whom he had no contact before the abduction.
Toluene is a clear, colourless liquid with a distinctive smell. It occurs naturally in crude oil and in the tolu tree.
It is also produced in the process of making gasoline and other fuels from crude oil and making coke from coal.
Toluene is used in making paints, paint thinners, fingernail polish, lacquers, adhesives and rubber, and in some printing and leather tanning processes.
It is extremely hazardous, even at low to moderate levels, affecting the nervous system, causing fatigue, weakness, confusion, memory loss, nausea, vision loss and hearing loss. At high levels it can cause intoxication, unconsciousness and death.