Students mourn in front of the Albertville school in Winnenden, Germany, Thursday after a 17-year-old burst into classrooms there Wednesday and gunned down students in a rampage that ended with 15 dead, authorities said.

WINNENDEN, Germany — The composed principal of a German high school was able to warn teachers with a pre-arranged code over the public address system when a gunman burst into the school, likely preventing the teenager from killing any more than the 12 students and teachers slain there, media reports said.

After the former student, identified by police as 17-year-old Tim K., 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 said.

"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." Germany's Bild newspaper reported the coded alert was worked out by German educators after a deadly school shooting in Erfurt in 2002 as a way to warn teachers.

Local media have identified the gunman as Tim Kretschmer and the name on his parent's home was Kretschmer.

After he escaped from the school Wednesday, 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.

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?"

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.

Authorities still have not given any indication of the gunman's motive. 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.

So far, police say they have found no letters or other written communication from the shooter indicating what provoked the attack.

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