Police announced Sunday that they had thwarted a plot by the pair to attack their school in Cologne after classmates saw them studying pictures of the Columbine school shooting and alerted their principal.
Police have said that one of the suspected plotters, a 17 year old, died when he threw himself in front of a tram after being questioned on Friday, and that the other, an 18 year old, was arrested and confessed to the plot.
On Monday, Cologne prosecutor Alf Willwacher said investigators have determined, through questioning and analysis of the boys' computers, that the elder boy backed out of the plot four weeks ago.
Prosecutors then decided against seeking an order to keep the 18-year-old in custody. Instead, he was admitted with his consent to a psychiatric clinic.
Officers have said they believed the pair had planned to storm the school with two crossbows and two air guns, which officers discovered during a search of the suspects' homes.
Willwacher said, however, that the elder teen had demanded that his suspected accomplice return the arrows from one of the crossbows. At that point, the 17-year-old got cold feet and said he would not carry out the plan alone, he added.
Neither teenager has been identified.
On Sunday, investigators said that, during a search of the teens' homes, police also found a list of names of teachers and students who were possibly to be targeted.