GERMAN police are searching for a Tunisian suspect in the Berlin truck attack whose immigration papers were found inside the vehicle, regional newspaper Allgemeine Zeitung reports.
The suspect is thought to be a Tunisian citizen known as Anis A., born in 1992 in the city of Tataouine, according to Speigel Online.
He is believed to be aged either 21 or 23 and is known to have three alias names.
Investigators have requested a public prosecution at the prosecutor's office, and preparations are underway. The Federal Criminal Police Office has announced a press conference at 1pm local time.
Security sources have told news agency DPA that police are planning an "imminent" operation in the German state of North Rhine-Westphalia related to the attack.
After prosecutors released a man arrested near the scene of Monday night's attack, saying there was insufficient evidence against him, police were racing to identify who was responsible for the attack which killed 12 people and injured 48 others at the market at Breitscheidplatz, near the famed Berlin Zoo.
The person responsible is thought to have access to a firearm, used to kill the Polish truck driver whose semi-trailer was hijacked earlier on Monday.
Jihadist group Islamic State have formally claimed responsibility for the attack, however mystery surrounds the truck lorry's final moments before it ploughed into a crowd of people.
An investigator reportedly told Bild News there "must have been a fight" despite initial reports the Polish truck driver had been killed before the terror attack.
Bild reported the terrorist drew a knife and stabbed the truck driver several times while he was attempting to halt the attack, however this is yet to be confirmed by authorities.
Meanwhile, The Sun reported German police had "botched" their hunt for the killer by arresting a man who had jumped a red light.
The innocent Pakistani refugee was taken into custody after being singled out for the minor traffic offence a mile down the road from the atrocity, the news agency reported.
And today as sheepish police conceded they have given the real killer an 18-hour head-start, they admitted: "We cried hurrah too quickly."
Police and intelligence agencies are trying to work out whether the attack was carried out by a "lone wolf'' single attacker, or if he was supported by others.
Federal criminal police chief Holger Munch said police were on "high alert.''
"We need to work on the assumption that an armed perpetrator is still on the loose," he said.
"We ... are investigating every possible angle."
Federal prosecutor Petter Frank told a press conference that it wasn't clear if the man was acting alone or as part of a group.
The man is known to have fled the truck after it came to a standstill in the market, pulling a mask off his face as he ran.
While a 23-year-old Pakistani asylum seeker was arrested near the scene and thought by police to be the main suspect, he has since been released after police reportedly failed to find any DNA or gunshot residue linking him to the truck.
Berlin police said they had received 508 leads to their tip-line.
Investigators were trawling through hundreds of photographs and videos taken by survivors at the market on their mobile phones.
Berlin police revealed last night that half of those injured had been released from hospital.
"We are full of hope, 24 injured people were able to leave hospital again and return to their relatives,'' they tweeted.
But there are concerns for several others who were critically injured when the fast-moving truck smashed through the market, with Interior Minister Thomas de Maiziere warning: "we cannot rule out the possibility that there will be more victims''.
In Italy, fears were growing for 31-year-old woman Fabrizia Lorenzo, who lives in Berlin but has has not been heard of since Monday night.
Her mobile phone and public transport card were found at the scene and Italian media reported her brother and sister had flown in to Berlin to provide DNA samples to police.
It is believed six of the 12 dead are Germans, while the death of Polish truck driver Lucasz Urban has been confirmed by his family.
An Israeli woman is also missing feared dead after attending the market on Monday night. Media reports say she was a tourist visiting with her husband, who was severely injured.
While jihadists had been claiming credit online for the attacks, it was not until late Tuesday night that the so-called Islamic State formally claimed responsibility.
In a statement through their Amal news agency, the extremist group said: "A soldier of the Islamic State carried out the Berlin operation in response to appeals to target citizens of the crusader coalition countries.''
Unlike the US, France, Jordan and Australia, Germany is not a part of the coalition striking Islamic State targets in Syria and Iraq but does provide logistical support from Turkey.
The market square in Breitscheidplatz remains cordoned off, with police armed with machine guns guarding the streets, and white hoarding closing off the area where the truck caused the most carnage, smashing through stalls, tables and chairs and striking market-goers as they shopped, ate and drank.
Makeshift shrines of candles and flowers remain in place around the square. The famed Brandenburg Gate was lit in the colours of the German flag overnight.