Dressed in black combats, Kretschmer had entered the school at about 9:30am [08:30 GMT] and opened fire, killing nine students aged between 14 and 15, and three teachers.
A passer-by died after the teenager stole the car, and two passers-by died in the final shootout between Kretschmer 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, a local police chief said the teenager was "constantly reloading his weapon".
|A local police chief said the shooter was "constantly reloading his weapon" [Reuters]
Angela Merkel, Germany's chancellor, 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."
Mark Seddon, Al Jazeera's correspondent in Winnenden, said: "There are many grieving parents. It's a very, very sad scene.
"He [Kretschmer] was a loner, he didn't graduate. 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," Wischgoll reported.
"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".
"He fired without any targets, he just fired into the classrooms and hit the students and teachers," Becker said.
After Kretschmer fled, he shot a passer-by before his final shootout with police, where two passers-by were killed.
Jumping from windows
Witnesses said students jumped from the windows of the school building after the teenager opened fire.
N-TV, a German television station, quoted a witness as saying: "The gunman just sprayed bullets all around him."
The TV station said the teenager was known to police.
The attack is 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.