Kretschmer returned to his former high school on Wednesday to kill nine students and three teachers before fleeing on foot and by car, killing three more people, and eventually turning a 9 mm Beretta pistol on himself after a shootout with police.
A joint statement released late Thursday by regional police and Stuttgart prosecutors said that, "in the course of the afternoon, doubts arose about the veracity of the Internet chat."
Police spokesman Klaus Hinderer said a search of Kretschmer's computer had shown no trace of his having made the chat room posting.
A message posted Thursday to the site - krautchan.net - said, "No killing spree was announced here." Prosecutors said they were trying to reach the U.S.-based provider of the site.
Across Germany, government buildings lowered their flags to half staff Thursday, while schools held moments of silence for the victims. Germany's national soccer league, the Bundesliga, said players would wear black armbands in upcoming games.
In Winnenden, hundreds of people filed into a church in a drizzle after dark for a prayer service. A crowd of many more watched on a large video screen outside as a message of condolence from Pope Benedict XVI, a fellow German, was read aloud.
Throughout the day, students and residents paid homage to the dead outside the school, where they lit candles and laid tulips, roses, handwritten notes and stuffed animals in a memorial.
Fourteen-year-old Kristin Puengel said a friend of hers was among the eight girls killed. Three female teachers and a boy were also shot in the school. Another three men were killed as Kretschmer fled police.
She said she only knew Kretschmer - who appeared in pictures shown on German television to be a dark-haired teen with glasses and short sideburns - by sight, but that he was not a friend.
"He was somewhat withdrawn, but I would never have thought (he would be capable) of anything like this," Puengel said.
Authorities and friends said that although Kretschmer played table tennis and lifted weights, his main hobbies appeared to be shooting and spending hours on his computer - where investigators said they found pornographic films, violent computer games and a collection of horror and action films that included "Rambo First Blood," "Freddy vs. Jason," and "The Marksman."
Officials said he had been interested in a girl of about his own age, but that the feelings were apparently never reciprocated. "It didn't work out," said Ralf Michelfelder, police chief in the nearby town of Waiblingen.
Kretschmer's father was a well-off businessman who legally owned 15 weapons and belonged to a gun club where his son regularly turned up for target practice, said Baden Wuerttemburg state Interior Minister Heribert Rech.
"He was well-trained in firing weapons," Rech said of the teen.
The teen, who graduated from Albertville high school with average grades in 2008, underwent several treatment sessions for depression at a psychiatric clinic that year, said investigator Siegfried Mahler. He showed up for five sessions between April and September, and was expected to continue outpatient therapy at a different clinic, but never began it, Mahler said.
Although authorities said he had struggled in school, Kretschmer was studying sales at a vocational school.
Authorities said they found some 60 shell casings in the school and that the number of victims could have been much higher had educators and police not carried out a plan learned in an earlier training program preparing them to respond to such a shooting.
When the first police squads arrived minutes later, they immediately stormed the building, under fire from Kretschmer as they came up the stairs before chasing him from the building, said state police president Erwin Hetger.
"We know from previous school shootings that the perpetrators only stop when they run out of ammunition, when they feel threatened by the police, or when they take their own lives," Hetger said.
"This gunman had more than 250 bullets on him when he entered the school," Hetger said.
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Melissa Eddy reported from Berlin.
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