Austrian police say four people have been arrested over the gruesome discovery of 71 bodies in an abandoned truck, with the victims believed to be Syrians.
A toddler, three young boys and eight women were among the dead in the latest horrific tragedy in Europe's unrelenting migrant crisis.
Another 76 bodies were recovered in the Mediterranean off Libya after yet another boat crammed with migrants sank, while a Swedish coastguard ship docked in Sicily carrying a grim cargo of 52 dead.
Austrian police spokesman Hans Peter Doskozil said Syrian travel documents were found in the truck abandoned on a motorway near the Hungarian border, suggesting the group were "likely" Syrians.
"Among these 71 people, there were 59 men, eight women and four children including a young girl one or two years old and three boys aged eight, nine or 10," he told a news conference.
A court on Saturday is to decide whether the four - three Bulgarians and an Afghan, according to Hungarian police - can be detained beyond an initial 72-hour period.
Those arrested included the owner of the vehicle and two drivers, and were likely "low-ranking members... of a Bulgarian-Hungarian human-trafficking gang", police spokesman Hans Peter Doskozil told a news conference.
Austria's public prosecutor Johann Fuchs said he would likely seek to have the suspects extradited.
Austrian motorway maintenance workers first saw the 7.5-tonne refrigerated poultry truck and noticed fluids dripping from the vehicle, Doskozil said.
Police were confronted by a mass of tangled limbs in the truck and forensics experts worked all night to clear out the vehicle, which had Hungarian plates.
The state of the corpses suggested that those inside had been dead for some time. Television images showed flies buzzing around the back of the vehicle in the baking sun.
German Chancellor Angela Merkel, in Austria for a summit with Balkan leaders on Europe's migrant crisis on Thursday (local time), said all those present was "shaken" by the "horrible" news.
"This is a warning to us to tackle this migrants issue quickly and in a European spirit, which means in a spirit of solidarity, and to find solutions," she said.
"Today is a dark day ... This tragedy affects us all deeply," Austrian Interior Minister Johanna Mikl-Leitner said.
Mikl-Leitner vowed to crack down on the people who pocket exorbitant sums to arrange migrants' passage to Europe, and then often leave them stranded en route.
"Human traffickers are criminals," she said.
Austrian newspaper Kurier carried a black front page with the headline: "Who will stop this madness?"
European Union leaders have struggled to get to grips with a crisis that has seen nearly 340,000 migrants cross the bloc's borders this year – not counting August – many from hotspots like Iraq and Syria.