Cantonal police have so far been baffled by the case, dating from December 21st 2015, when a 48-year-old woman, her sons aged 13 and 19 and and the 21-year-old girlfriend of the elder son were found dead in a home at Rapperswil that had been set on fire.
The victims were bound with cables and stabbed to death before the fire was set, according to information released earlier by police.
Investigators have established that the mother made two withdrawals from separate bank ATMs in the last few hours of her life, when she took out more than 9,850 francs and €1,000.
But on Thursday, Aargau's attorney general Philip Umbricht admitted at a press conference on Thursday that other than that police had no serious leads in the case.
This was despite the fact they had received around 250 tips from the public, interviewed 110 people and examined DNA evidence.
A task force of 40 police investigators have been working on the case
Aargau authorities decided to offer the reward in the hopes of getting fresh information that might cast light on what happened.
“It is one of the biggest rewards ever offered in Switzerland,” Umbricht told the press conference, the ATS news agency reported.
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The crime is one of the most serious crimes the canton has had to face, he said.
“At this stage every detail can prove to be a piece of the puzzle.”