BERLIN—German investigators said fingerprints found in the truck used in this week’s terror attack pointed to the central suspect as the likely perpetrator, as it emerged that Germany had alerted the U.S. in June that the 24-year-old Tunisian asylum seeker may pose a risk.

The new evidence in an investigation that otherwise showed little progress in the fourth day since the attack came as the government hit back at accusations that authorities failed to stop Anis Amri, the fugitive suspect, despite being aware of his radicalization...