The father of two missing Swiss twin girls trawled websites on suicide, guns and poison before killing himself, police say.
Police also said that Matthias Schepp, who was last seen with the twins on a ferry to the island of Corsica, made the trip back to mainland France alone.
The uncle of six-year-old Alessia and Livia said the latest news was "very, very worrying".
Schepp threw himself under a train in southern Italy on 3 February.
Police spokesman Jean-Christophe Sauterel said analysis of Schepp's work computer had turned up the websites, which also included ferry schedules.
"These factors show that the father had carefully planned his journey," Mr Sauterel said.
"The investigation is now focusing on trying to establish more precisely the movements of the father from Tuesday noon, when he arrived in Corsica with the girls, until Thursday noon, when he was in the region of Naples."
Meanwhile, police official Alfredo Fabbrocini told Associated Press that Schepp was on his own when he sailed back to the port city of Toulon in mainland France from Corsica on 1 February.
Prosecutors earlier this week said that the girls were last seen with their father on a ferry to Corsica - the first confirmation that they had boarded the boat.
The 43-year-old father had been looking after the girls for the weekend, but failed to return them home as planned.
He picked up the girls from his estranged wife's house in the Swiss village of St Sulpice on 28 January, before travelling to France and Italy. He was found dead in Cerignola, in the region of Puglia.
An unconfirmed report from a cafe owner in Cerignola emerged in recent days suggesting that the girls were seen with their father shortly before he threw himself under a train.
But Mr Fabbrochini said police had viewed closed circuit video footage from the cafe "over and over," but had not seen them.
He added that days of searching the Cerignola area with sniffer dogs had failed to turn up any trace of the girls.
The twins' uncle, Valerio Lucidi, said Schepp's "terrible premeditation" was "very, very worrying", according to AFP news agency.
On Wednesday, the girls' mother, Irina Lucidi, went on Italian TV to urge viewers who might know anything about the case to contact police.
"I appeal to whoever has seen them or knows something to contact the police," she said on Rai 3's primetime news bulletin.
Alessia was dressed in blue jeans, a striped T-shirt and a white jacket, while Livia wore a purple ski jacket with white and pink sneakers.