Five boys, aged between 16 and 18-years-old, have been arrested and several guns and others weapons are reported to have been seized.
The alleged plot to attack Riverton High School came to light after a message was posted on the popular MySpace.com website by one of the boys.
According to police the message alluded to an attack on the school on April 20 and warned staff and students to wear flack jackets on that day.
It also discussed the significance of the date - the anniversary of the Columbine school shooting and the birthday of Adolf Hitler.
The boys were all arrested on Thursday, the day the alleged plot was supposed to have been carried out.
None of the names of the teens, ages 16-18, were released.
Sheriff Steve Norman said: "What the resounding theme is: They were actually going to do this."
"The sheriff's office believes it is the real thing. I have no reason to mistrust their judgment"
Superintendent David Walters
Norman added that guns, ammunition, knives and coded messages had been recovered from the bedroom of one suspect.
He said documents about firearms and references to Armageddon had also been found school lockers belonging to two of the suspects.
Police interviews with boys had indicated they planned to wear black trench coats and disable the school's camera system before starting the attack between noon and 1pm on Thursday, Norman said.
Nathan Spriggs, 15, a friend of the arrested boys, said they had told him that they had posted a threat on the internet as a joke and feared they would be suspended or expelled for doing it.
But school officials and police were taking no chances.
"They weren't bullied a whole lot. They bullied a few kids"
Superintendent David Walters, said: "The sheriff's office believes it is the real thing. I have no reason to mistrust their judgment."
The suspects had apparently been plotting the attack since the beginning of the school year.
Brandon Hay, 18, said, one of the suspect has a history of school fights and had made such comments as "I wish so and so died" or "I am going to shoot someone."
Hay said: "We knew it was joking around. They weren't bullied a whole lot. They bullied a few kids."
Leah Virgil, a mother of two students who knew the suspects, said they were "loners, misfits" who had "a lot of anger".
General Phill Kline, a Kansas lawyer whose office took over the prosecution at the request of the county lawyer, said charges are likely to be announced on Friday, when the suspects are expected to appear in court.
On April 20, 1999, two teenage students, Eric Harris and Dylan Klebold, went on shooting ramage at Columbine High School in Colorado, killing twelve fellow students and a teacher before committing suicide.
Two students killed thirteen people
at Columbine High School in 1999
Twenty-four others were wounded in the attack.
Since the Columbine massacre, many schools in the US have taken tighter security measures including airport-style metal detectors at school entrances, airport-style metal detectors and random bag searches.