Kretschmer killed 12 people in the school and three others in the nearby town of Wendlingen. He then shot himself after a shoot-out with police.
Police say chatroom users did not take the comments seriously at the time. At a news conference, Baden-Wurttemberg's Interior Minister Heribert Rech said the gunman had spoken of his attack in a chatroom on a German internet portal.He said the message read: "I've had enough. I'm fed up with this horrid life... Always the same".
"People are laughing at me... No-one sees my potential... I am scared, I have weapons here, and I will go to my former school tomorrow and then I will really do a grilling."
The message then continued: "Possibly I get away, so keep your ears open, you will hear from me tomorrow. Just remember the name of the place, Winnenden."
Mr Rech said a German man alerted police about the internet warning after the school shooting.
The man said his teenage son told him about the warning only after seeing the news reports. He had not previously taken the threat seriously.
Psychiatric care Officials say Kretschmer fired more than 100 shots during Wednesday's attack on his former school.
Nine students - eight of them girls - and three teachers died at the Albertville secondary school, many of them shot at close range.
Mr Rech said students appeared to have had little time to react.
"When their bodies were later found, some of them still had pens in their hands," he said.
Germany's Bild newspaper reported students as saying he had gone into one classroom three times, asking students: "Aren't you all dead?"
Hans-Dieter Wagner, police director for the Esslingen area, told the news conference Kretschmer had fled the school on foot.
In the following three hours, he injured a passerby and shot dead an employee at a psychiatric clinic. He then shot and killed an employee and a customer at a car showroom in a nearby town.
Officials say he still had more than 130 rounds of ammunition left when he was cornered by police and shot himself. The officials also revealed that Kretschmer received psychiatric care for depression in 2008. They said he was meant to continue an out-patient treatment but refused.
However, investigators said they could not speak of a connection between that treatment and Kretschmer's rampage.
Prosecutors say they may charge Kretschmer's father with failing to secure the gun used by the teenager in the attack.
German Interior Minister Wolfgang Schaeuble said he could not see how a change in the country's weapons laws would have prevented what happened.
"We are checking everything but our arms law is very strict," he said.
But he said Germany had to consider whether tighter controls on access to violent imagery were needed.
Flags are flying at half-mast across Germany on Thursday as a mark of respect for the victims of the shootings.




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