A spokesman for Waiblingen police said there was no record on the Winnenden youth's computer that he had logged into the chat forum on the eve of the Albertville school massacre, but were questioning witnesses who reported seeing the entry to clarify if and when such a threat was made.
"Doubts emerged during the afternoon about the veracity of the entry in the Internet chat room. Of course every piece of information, especially concerning this entry, is being vigorously examined," police said in a statement.
The chat room www.krautchan.net, which Tim K. had allegedly used, posted a statement denying the claim. The Web site said that the press had been "unfortunately fooled... by a forgery."
"No killing spree was announced here," it said. "Maybe he visited the site, but he definitely didn't write the post that went through the news, because that one never existed."
German authorities are now working with the US Internet company that hosts the online forum's servers to find out whether the gunman actually posted the threat.
On Thursday, Heribert Rech, the interior minister of Baden-Wuerttemberg, announced at a press conference that the teenage gunman had given a clear warning about his deadly intentions in comments posted in a chatroom.
"I've had enough," Rech quoted from the chatroom message. "Always the same. Everybody's laughing at me. No one sees my potential. I'm serious. I have weapons and I will go to my former school in the morning and have a proper barbecue. Maybe I'll get away. Listen out. You will hear of me tomorrow. Remember the place's name: Winnenden."
Rech said Tim K's interview partner had not taken the threat seriously at first but he told his father about it after seeing reports of the shootings.
In a newspaper interview on Friday, Rech moved to clarify his position on the statements.
"Some crazy person obviously put out this dreadful false message," Rech was quoted as saying. "It must have been made up afterwards."
"I always made clear that I was referring to preliminary findings of the investigation. It must now be cleared up how the father of a 17-year-old could claim to have seen the entry," Rech said.
Less than seven hours after the alleged conversation, at around 09:30 CET, the teenage gunman entered the Albertville school, armed with a handgun taken from his father's bedroom and more than 200 rounds of ammunition.
He fired 60 bullets at the school, killing eight girls, one boy and three female teachers, mostly with shots to the head. He then fled, hijacked a car and randomly shot dead three bystanders.
Three hours later he was dead after a manhunt ended in a police shootout 30 kilometers (20 miles) away. State police chief Erwin Hetger said it was believed he had turned the gun on himself.
Later Thursday, a grainy amateur mobile phone video, apparently showing the killer's dramatic last moments and death, surfaced on the Internet.
The two-minute clip shows a figure wielding a gun in a car park as shots ring out around him. He then suddenly falls to the ground and seconds later is surrounded by dozens of police officers in green uniforms.
Hoaxers arrested across Europe
In the wake of the horrific events in the small town, a total of four people have been arrested for allegedly posting Internet warnings of school shootings.
German police in Lower Saxony said they arrested a 21-year-old man who boasted plans to carry out a similar crime in a chatroom.
"I have a gun and I'm going to kill everybody," read the message.
The man, who claims the message was "a joke," faces up to three year in jail or a fine.
French police arrested an 18-year-old after he also posted a similar threat on a Web site. "In Raincy (high school) there will be blood -- I have weapons" the message read.
Dutch police also nabbed an 18-year-old for a similar warning and on Thursday Swedish police took a 17-year-old in for questioning for allegedly posting a picture of himself posing with a weapon on an Internet forum, along with a threatening message against a high school.
Gunman was treated for depression
As hundreds of people attended a church service for victims of the school massacre on Thursday, more details of the gunman's life began to emerge.
His father is a successful businessman who employs 150 people at a packaging firm, according to police, but his son found it difficult to fit in at school and had few friends.
"He was simply not accepted by anyone and just sat all day in front of his computer," Mario, a schoolmate, told German television station N24.
Reports also said he was keen on computer shooting games -- especially the violent "Counter-Strike" -- and had become a real-life crack shot at the shooting range where his father was a member.
After leaving school last year, Tim K. enrolled on a course to train as a salesman. He regularly worked out at the gym, belonged to a sports club and was a keen table tennis player.
After ending his studies last year, Tim K. spent time in hospital undergoing treatment for depression. He was supposed to continue his treatment at a psychiatric clinic adjacent to the school but broke off the therapy, officials said.
Guns appeared to be part of his life. His father owned more than a dozen weapons, all locked away except the nine millimeter Beretta pistol that he used in the shootings. Police also found 4,600 rounds of ammunition at the house.
Officials said Tim K. had apparently cracked an eight-digit code to a locked cabinet containing guns and ammunition.
Police said the father might have to face charges of involuntary manslaughter because he had failed to lock up the weapon. The parents, who also have a 15-year-old daughter, moved out of the family home on Wednesday to escape media attention.
The Albertville school remained closed as psychologists provided counseling to traumatized students and teachers as well as the families of the victims.
Fears of copy-cat killings
Officials said there had been threats to six other schools in Baden-Wuerttemberg after Wednesday's shooting, prompting police to increase surveillance of school buildings.
The Frankfurter Allgemeine Zeitung newspaper echoed fears that the tragedy was almost certain to inspire others to emulate it.
"The images of the rampage encourage politicians to voice their outrage, shock and how dumbfounded they are. It encourages the police to fine tune their video simulations and role-playing for such massacres," the paper wrote. “Why shouldn't the images of these massacres encourage all kinds of people but not the next person set to cause another massacre?”
The shooting was Germany's worst school massacre since April 2002, when a 19-year-old high school student went on a rampage in Erfurt, killing 12 teachers, two students, a school secretary and a policeman before killing himself.
It has also renewed the debate on whether Germany should tighten its gun control laws and install metal detectors at schools.
Media call for heightened school security
German Interior Minister Wolfgang Schaeuble told Reuters he saw no need to tighten gun controls further after the shooting. "We shouldn't think about tougher laws all the time, but think about what we can change in society," he said.
The Sueddeutsche Zeitung argued that stricter gun laws wouldn't prevent similar tragedies, but suggested that increased checks on current gun owners would be a good place to start. "Even owning a weapon is tied to several obligations," the paper wrote. "The father of the deadly shooter did not fulfill these and therefore he'll have to be held accountable."
Conservative broadsheet Die Welt called for security to be increased in Germany's schools. "At every airport and at many companies we are subject to search, but anyone can enter a German classroom unchecked."
It added that the nation much look at what motivated Tim K. in order to keep similar outrages from happening again. "Prevention focused on perpetrators must be broadly expanded so as to see early warning signals," the paper commented.
Der Tagesspiegel claimed the teenage gunman was a product of his environment and German society, which failed to offer him the support and help he needed to deal with his inner turmoil.
"The school, his friends and his parents, didn't offer him this chance, which is every child's right. Now, as the culprit will have calculated, he is taken seriously. The price to pay for a failure like this is unacceptable," the paper wrote.