Germany is still in shock after 19-year-old Robert Steinhäuser (photo) ran amok in his former school on Friday.

The student had recently been expelled from Gutenberg High School. On Friday, he shot and killed 13 teachers, two students and a police officer before killing himself.

But as police and psychologists are still trying to piece together Robert Steinhäuser's motives, the chronology of Friday's shooting is becoming clearer. This weekend, details emerged that the courageous intervention of a teacher ended Steinhäuser's killing spree.

60-year old history and arts teacher Rainer Heise confronted the heavily armed gunman, engaged him in a conversation and managed to lock him in an empty classroom.

Teacher confronts the killer

Rainer Heise was in the midst of teaching sixth graders about art when he heard a noise in the school on Friday morning. "Suddenly I heard a loud bang," Heise told Germany's Bild am Sonntag Sunday-newspaper. "It sounded as though chairs were being thrown around."

At first Heise thought there had been an accident in the school's chemistry lab. As he left his classroom, he saw students running along the school corridors. Suddenly, he was face to face with a man all clad in black.

Heise says the man was wearing a face mask, wielding a gun and shooting into the air. Then the gunman shouted "Damned, I've got to reload" and extracted some bullets from a pocket in his pants.

As the gunman was reloading his pistol, Heise ran to the school secretariat. He shouted and banged on the door, but it was locked. After a short moment, the school principal opened and let him in. "There are dead people here," she told Heise in a trembling voice.

When Heise entered the secretariat, he saw the assistant principal sitting at her desk – shot dead. The school secretary was lying on the floor. She too had been killed.

Heise suggested to the principal that he would check if all students had gotten out of the school building. He left the secretariat to look into the classrooms on that floor.

As he checked one of the rooms used for art classes, Heise opened a window and quickly passed on information to the police forces that had amassed outside: "We need four ambulances," Heise shouted. "The gunman is still in this building," he added.

The policemen told Heise to get away from the window. Heise then locked himself into the room to keep the gunman out.

Courageous words end the violence

After a little while, the 60-year-old teacher heard a sound outside the door. He thought it might be a student who hid from the killer somewhere. He opened the door to peek out into the hallway. Right in front of him was the masked gunman, pointing his gun directly at him.

"Then all of a sudden he takes off his face mask," Rainer Heise continues. The teacher realizes the gunman is his former student Robert Steinhäuser. "I ask him: Robert, what's all this about? Was that you shooting?"

Rainer Heise looks at the 19-year-old and calmly says: "'If you're going to kill me, look me in the face. Then he looked at me, dropped the gun and said, 'No, Mr. Heise, that's enough for today,'" the teacher explains.

Oddly, the former student, who had at that point already killed 16 people, replied to Heise in polite and formal language. He respectfully referred to the teacher as "Herr Heise".

Teacher locks former student in empty classroom

Rainer Heise said he then made Robert an offer to talk about what he had done. He made a gesture inviting the disgruntled former student to enter the classroom. As Steinhäuser stepped in, Heise seized his opportunity. "I pushed him into the room and locked the door," Heise said.

Shortly thereafter, as police forces were moving in on him, Robert Steinhäuser shot himself in the head.

Police said late on Saturday the teacher's bravery had prevented an even higher death toll. "A courageous teacher involved the assailant in a discussion and most probably prevented a far worse massacre," said police spokesman Manfred Etzel in Erfurt.

Heise has been hailed a hero in Germany.

The police are now trying to piece together just what drove Robert Steinhäuser to run amok. They have already established that the 19-year old had no previous criminal record. He was a member of two separate gun clubs, and had obtained his weapons legally.

Police say it appears as if Robert Steinhäuser had been planning the attack for a long time - at total some 1200 rounds of ammunition were found on his body and in the apartment he shared with his mother.