SPOKANE, Wash. -

Spokane police are crediting students and WinCo grocery store employees with disrupting a possible a threat to attack North Central High School later this month.

On Wednesday the 16-year-old suspect, who has not been named, appeared in juvenile court where a judge placed him under house arrest.

Court documents allege the teen was overheard talking about blowing up propane tanks inside the school and then taking a gun and start shooting people.

Detectives didn't know who their suspect was until he allegedly warned of the impending attack with some graffiti. The break officers were hoping for came on April 10 when employees at WinCo Foods noticed new graffiti inside the men's bathroom.

Someone had written on the wall "I'm serius [sic]. Nc school shooting 4/20/15 be ready."

April 20 is the anniversary of the Columbine High School massacre.

"In this case, thankfully, a citizen was able to look at the graffiti and realize this isn't just your run of the mill graffiti, there's something extra here and rather than dismiss it he took the extra step," Major Craig Meidl of the Spokane Police Department said.

WinCo employees called police and volunteered to provide the arriving officer surveillance video of a teen seen using the bathroom several times earlier that morning. The store also printed out a photo of the teen, which was shown to North Central's district resource officer, who recognized him as the suspect who was arrested on a threats to bomb or injure charge Tuesday.

"As a police department we considered ourselves a big piece of keeping those schools safe and in working with District 81 we were able to in this case avert what may have possibly been a tragedy," Meidl said.

In court however, the student's public defender pointed out her client wasn't threatening anyone but rather warning about something he had heard about on the street.

Prosecutors wanted the teen detained until he could receive a mental health evaluation but Judge Ellen Clark ruled there wasn't enough evidence to warrant holding him behind bars

"I can understand why the school is upset and why they have expelled him and certainly should but as for what we are going to do today I don't think I have a legal basis to detain him," Judge Clark said.

However Judge Clark did place the student under house arrest and as a condition of release has to stay away from his former high school.