12 people died after a truck drove through Breitscheidplatz
The suspect arrested over the Berlin Christmas market attack may not the person responsible, German authorities have admitted.
12 people died when a lorry crashed into a crowded Christmas market.
Berlin's police chief, Klaus Kandt, told reporters: "As far as I know it is in fact uncertain whether that really was the driver."
Prosecutors also said they were unsure if they had the right person in custody and admitted he could be one of a group.
Fears that the perpetrator could have escaped have been fuelled by the fact that the gun used to shoot the Polish driver of the lorry has not been recovered.
Newspaper Die Welt said a senior police source had told them: "We have the wrong man and therefore a new situation.
"The true perpetrator is still armed, at large and can cause fresh damage."
The temporary arrested suspect denies the offense.— PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 20, 2016
Therefore we are particulary alert.
Please be also alert.#Breitscheidplatz
Police also say they are continuing the investigation "with emphasis".
Interior minister Thomas de Maiziere said it was carried out by a migrant who was probably from Pakistan.
He said there was "no doubt" it was a terror attack - but that the person arrested had denied involvement.
Angela Merkel says millions of Germans are shocked and saddened after the suspected attack.
"As millions of people in Germany, I'm shocked and very saddened by what happened last night at the Breitscheidplatz in Berlin," Mrs Merkel said.
"12 people that were amongst us yesterday, that were looking forward to Christmas and their plans for the festive season, are no longer amongst us.
"It is a terrible deed which one cannot understand."
It is being reported the suspect is a 23-year-old migrant from Pakistan. Mrs Merkel says if this is true, it would be 'terrible'.
"We don't have anything for certain, but we must assume it was a terrorist attack.
"It would be very difficult for us to learn that a human being committed this deed who came to Germany to ask for refuge and asylum.
"It would be terrible for all of the Germans who are very active day by day in helping asylum seekers and refugees - it would be repugnant for them, for those that are helping people that have come to this country and are asking for our help".
Police are working on the assumption the lorry was intentionally driven into the Christmas market at Breitscheidplatz on Monday evening.
Witnesses described the truck smashing through the tourist spot - filled with huts serving hot wine and food - at around 8pm and flattening everything in its path.
It travelled as far as 80m before coming to a halt.
Dubliner Killian McDonagh has lived in Berlin for the past three years.
He told the Pat Kenny Show here on Newstalk the market was full of people on Monday night.
"We walked through the Christmas market where the attack occurred about 15 minutes beforehand.
"It's Berlin's main shopping district, especially this time of year, there are people all over the place doing their shopping...so it was really very packed."
Killan says this incident was almost expected.
"It's certainly from what I can see the best place to live in Europe - which is I suppose why it was such a target.
"People in Germany have been expecting this because Germany has kind of been the moral leader of the free world, and the lifestyle that people have here is envied all over the world.
"In many ways people were expecting this to happen and I suppose it's just good that it wasn't even worse than it was".
48 people were also injured in the suspected attack and Chancellor Merkel said some were fighting for their life.
The apparent suspect was arrested about a mile away from the crash.
A 37-year-old Polish man was found dead in the passenger seat.
The German interior minister said there was a gun wound, but that a post-mortem examination had not yet been completed.
The owner of the company which operated the truck said the driver - who was also his cousin - had been stabbed and shot.
He said he had identified him from police photos.
German media reported that analysis of GPS data by the lorry's owners found there appeared to have been several attempts to start it leading up to the incident - at 3.44pm, 4.52pm and 5.37pm.
It finally got under way at 7.34pm before careering into the market at around 8.00pm.
Foreign Affairs Minister Charlie Flanagan is urging Irish citizens in the city to make contact with their families.
In a statement, he also said he was "horrified" at the events in the German capital.
While anyone here worried about Irish citizens in Berlin can call the Department of Foreign Affairs on +353-1-408-2000.