Flags across Germany flew at half mast on Thursday, and hundreds of candles were left outside the school where the massacre took place.
People packed churches for special services on Wednesday night, while dozens attended a vigil outside the school.
Mark Seddon, Al Jazeera's correspondent in Winnenden, said "why?" is the question on everyone's lips in the small town of 27,000 people.
"Counsellors from all over the country are flying in to spend time with children and parents, to try and understand what on earth happened," he said.
"What made a young student go on a rampage killing fellow pupils, killing teachers -and what will people learn to try and ensure this doesn't happen again?"
No apparent motive
A video taken on a mobile phone showed Kretschmer at a car dealership where he killed three people, then himself.
There was no immediate indication of motive, but the victims were mostly female: eight of nine students killed were girls, and all three teachers were women.
Three men were killed later as the suspect fled.
The massacre occurred just hours after a man went on the rampage in the southern US state of Alabama, killing 10 people before also turning the gun on himself.
Speaking for a nation stunned by the crime, Chancellor Merkel said: "It is unimaginable that in just seconds, pupils and teachers were killed, it is an appalling crime. This is a day of mourning for the whole of Germany."
Wolfgang Schauble, the German interior minister, ordered flags across the country to be flown at half mast on Thursday as a mark of respect for the dead.
Dressed in black
Dressed in black combats, Kretschmer had entered the school at about 9:30am local time [08:30 GMT] and opened fire, killing nine students aged between 14 and 15, and three teachers.
A passer-by died after Kretschmer seized the car, and two passers-by died in the final shootout between him and the police.
Erwin Hetger, a regional police chief, said: "He went into the school with a weapon and carried out a bloodbath. I've never seen anything like this in my life."
Konrad Gelden, the local police chief, said the teenager was "constantly reloading his weapon".
Mark Seddon said there were "many grieving parents. It's a very, very sad scene.
"He [Kretschmer] was a loner. ... He had an interest in the occult and his father was a member of a gun club. They had 16 guns in the house.
"He left no message and so far there appears to be no motive.
"People are full of praise for the swift reaction of the police ... the death toll could have been higher. The police found other munitions in the grounds so this was clearly planned."
Petra Wischgoll, a journalist in Cologne, told Al Jazeera that police believe the suspect had finished school last year.
"He was always a very quiet student, who never did anything, and wasn't really big in the picture," she said.
|A local police chief said the shooter was 'constantly reloading his weapon' [Reuters]
"He was very quiet and nobody ever thought of him much. So everybody is very surprised that it was him doing the shooting."
Lothar Becker, a journalist with ZDF, a German broadcaster, told Al Jazeera the suspect was "a normal young guy, from a family with no financial problems, in an area with no criminal problems".
He said it was "probably the worst school massacre we ever had in Germany".
The attack is indeed Germany's worst school shooting since 2002, when 16 people were killed at a high school in Erfurt in eastern Germany by a 19-year-old former student, who then turned the gun on himself.
In November 2006, a former student at a vocational school in Lower Saxony in northwestern Germany went a shooting spree, injuring 37 people before killing himself.