Authorities have dropped charges against five Arizona high school students suspected of plotting another student's murder.

The Pima County Attorney's Office says there isn't substantial evidence to prove the students actually planned to carry out a plan to kill a fellow student at Flowing Wells High School in Tucson.

Police arrested the students on March 18, charging four who are juveniles as adults. The fifth student is 18 years old.

The students faced a count of conspiracy to commit first-degree murder after police said they spent a week planning to kill a student they were having problems with. One of the suspects had a knife at school.

Prosecutors say the student didn't have the knife for the purposes of killing. Still, that student could face a criminal charge for having a weapon on campus.