A teenage gunman who killed 15 people in a German school and nearby had given an internet warning before he went on a shooting rampage, German officials say.

They say Tim Kretschmer, aged 17, gave the warning in a chatroom before the killings in Winnenden on Wednesday.

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.

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.

WINNENDEN, Germany (AP) -- The 17-year-old gunman who went on a rampage at his former school and killed 15 people before taking his own life gave a warning in an Internet chatroom only hours earlier and said he was "sick of this life," officials said Thursday.

The suspect, whose name appears as Tim Kretschmer in police documents, told others in the chat room he planned to attack his school in Winnenden, said Baden Wuerttemburg state Interior Minister Heribert Rech.

Rech said the suspect wrote, "You will hear from me tomorrow, remember the name of a place called Winnenden."

In the first indication of a motive in the shooting, Rech said the teenager told others in the German-language chat room that: "Everyone laughs at me, nobody recognizes my potential."

"I'm serious. ... I have a weapon here," Rech said the youth wrote. "Early tomorrow morning I will go to my former school."

Rech said the chat had occurred the night before the attack, but a police official, Erwin Hetger, later said it was in the early morning Wednesday, about six hours before the 9:30 a.m. shooting.

The youth ended the chat saying, "No reports to the police now, don't worry, I'm just baiting you."

A Bavarian man told police about the chat after the school shooting in Winnenden had taken place, Rech said. He told authorities his 17-year-old son only told him about it after seeing the news reports and had not taken the threat seriously, replying to the message: LOL - shortform for "laughing out loud."

Investigator Siegfried Mahler said authorities had learned that the suspect was treated for depression in 2008, with five visits to a psychiatrist between April and September at an area clinic. He was supposed to continue the treatment at another clinic, but apparently did not show up, Mahler said.

Despite the high death toll, the shooting could have been worse if the principal of the high school had not been able to warn teachers with a prearranged code over the public address system when the suspect burst into the school.

After the suspect entered the school in Winnenden on Wednesday morning and opened fire, the principal put the emergency plan in effect, quickly broadcasting a coded message to teachers: "Frau Koma is coming," students told German media.

"Then our teacher closed the door and said we should close the windows and sit on the floor," a student, identified only as Kim S., told ZDF television.

In German the word "amoklauf" is used to describe school shootings, and "koma" is the reverse of the word "amok." Hetger said the coded alert was worked out by German educators after a deadly school shooting in Erfurt in 2002 as a way to warn teachers.

Hetger said that police in Baden-Wuerttemburg had received special training that involved sending small teams into the building in the event of a school shooting, as happened on Wednesday.

He credited this, coupled with the warning from the principal, for preventing further deaths.

Although the gunman shot at the officers as they entered the building, they succeeded in chasing him from the premises.

After he escaped, he hijacked a car and was eventually caught in a police shootout. The rampage ended with 15 victims slain and the assailant taking his own life, authorities said.

His victims were primarily female: eight of nine students killed were girls, and all three teachers were women. Three men were killed later by the suspect as he fled.

The dark-haired teen, shown wearing glasses in pictures on German television, apparently took the weapon from his father's collection of 15 firearms along with a "multitude of ammunition," police said. His father, a businessman, was a member of the local gun club and kept the weapons locked away except for the pistol, which was kept in the bedroom.

The high school was closed Thursday, still cordoned off by red and white police tape as investigators pored through the building. Scores of candles lit by mourners adorned the grounds amid bunches of flowers and notes with messages and questions like "Why?"

A man carried a sign saying, "God: Where were you?"

The government ordered all federal buildings to fly their flags at half staff, and schools across the country held moments of silence for the victims. Germany's national soccer league, the Bundesliga, said players would wear black armbands in upcoming games.

Injured student Patrick S., 15, was quoted by Bild newspaper as saying Kretschmer burst into his German class at about 9:30 a.m.

"We flipped over the desks to duck behind for cover. ... Suddenly I saw that I was hit - in the back, in the arm and in the cheek," he was quoted as saying. "Suddenly he was gone and we barricaded the door. And then I saw my classmate Chantal. She sat at the door. Dead."

Local police spokesman Nik Brenner said that authorities had found 60 shell casings in the school.

By Nicola Leske

WINNENDEN, Germany (Reuters) - German authorities are looking into whether to press charges against the gun-collecting father of the teenager who went on a shooting rampage at his former school on Wednesday, killing 15 people.

The motive for the attack by 17-year-old Tim Kretschmer remained unclear a day after the massacre in the southwestern town of Winnenden.

The teenager, who neighbors described as a loner with a fondness for violent videos, appears to have taken his own life after police engaged him in a shootout hours after he fled the school and hijacked a car.

Kretschmer had taken a legally-registered 9-millimetre Beretta pistol owned by his father, a member of a shooting club.

Police said the father's other 14 guns were locked in a gun-closet as required by German law, but that he kept the Beretta in his bedroom.

"Everything here points to negligence on the part of the father as far as the storage of this weapon is concerned," said police spokesman Ralf Michelfelder.

He said it was now up to local prosecutors to decide whether to press charges against the father.

Germany toughened its gun laws in 2002 after 19-year-old Robert Steinhauser shot dead 16 people, mainly teachers, and himself at a high school in the eastern German city of Erfurt.

The changes raised the minimum age for gun ownership to 21 from 18 and required gun buyers under 25 to present a certificate of medical and psychological health.

German gun control laws already required applicants to pass rigorous exams that can take up to a year, but there are close to eight million legally held weapons in the country, roughly one for every 10 people.

Schools were closed in Winnenden on Thursday but students streamed to the Albertville Realschule, scene of the shootings, with flowers, candles, CDs and stuffed animals. Some stood in silent grief, some cried and some clung to friends or parents.

Police said on Thursday Kretschmer had fired 60 rounds in the school after entering a classroom at around 9.30 a.m. (0830 GMT). He shot many of his victims in the head at close range and appears to have targeted women.

Eight of the nine students and all three of the teachers he killed in the school were female.

When police arrived at the school Kretschmer fled, shot another person in front of a nearby psychiatric clinic and then forced a motorist to drive him out of Winnenden, a town of 27,000 near Stuttgart.

He jumped out of the hijacked car in Wendlingen, some 30 km (20 miles) from the school, shot two men at a car dealership and severely injured two policemen before apparently turning the gun on himself.