The Berlin Christmas market terrorist circled his target three times and sent pictures and voice messages on a cellphone up to ten minutes before the Dec. 19 attack, Germany's Focus magazine reported Wednesday.
Investigators reportedly uncovered the details after searching Anis Amri's phone.
Authorities have sought eyewitness accounts and phone images from passersby due to an odd lack of surveillance cameras in the busy locale.
After driving his truck into a crowd at the market, killing 12 and injuring 48, Amri began a 1,200-mile trek to Italy that officials are still investigating. Amri journeyed through five European countries over several days -- all while he was the most-wanted man in Europe.
Amri, 24, was killed in a shootout with police in a suburb of Milan early Friday.
German prosecutors said Wednesday that they detained a Tunisian man they think may have been involved in the truck attack. Officials linked the man to Amri also by searching Amri's cellphone.
The 40-year-old, who wasn't identified, was detained during a search of his home and business, federal prosecutors said.
Prosecutors said in a statement that "further investigations indicate that he may have been involved in the attack."
Authorities have until Thursday evening to determine whether the case against the 40-year-old is strong enough for them to seek a formal arrest warrant. That would allow them to keep him in custody pending possible charges.
Investigators are trying to determine whether Amri had a support network in planning and carrying out the attack, and in fleeing Berlin.
Fox News' Greg Palkot and The Associated Press contributed to this report.