"A man corresponding to the killer was spotted on the afternoon of Thursday, December 22, on a platform at the (Lyon-Part-Dieu) station wearing a cap and backpack," the source said, confirming media reports. "He appears alone in these images."

The source said investigators are still trying to determine how 24-year-old Amri, suspected of using a hijacked truck to mow down 12 people at a Christmas market in Berlin a week ago, was able to leave the German capital to reach France and then Italy.

The Berlin rampage was claimed by the Islamic State group, which released a video on Friday in which Amri is shown pledging allegiance to IS chief Abu Bakr al-Baghdadi.

German authorities are probing whether Amri had help before or after the attack. Hundreds of investigators are set to work on the case throughout the holiday season.