When Chelsie Shellhas saw the young man fumbling with the lock at a Waseca, Minn., storage facility then enter and quickly shut the door, she became suspicious and called police.

“He shut the door, and I thought it looked funny,” Shellhas told WCCO-TV. “Because, normally, we see people come here, and it doesn’t take them 10 minutes to open a storage shed, so that’s why I called it in.”

Police responding to her call Tuesday found a 17-year-old inside the storage unit with a stockpile of equipment that included materials used for making explosive devices. At the teenager's home officers said they discovered a second stash of handguns, automatic weapons and a 180-page journal that detailed his plans to murder his parents and sister, then attack Waseca's junior and senior high school with pressure cooker-type bombs and Molotov cocktails, according to a criminal complaint filed on Thursday.

Police praised Shellhas’ quick action for preventing an “unimaginable tragedy” -- attacks on the school in the city of about 9,500 people, about 80 miles from Minneapolis. The suspect planned to kill “as many students as he could” to commemorate the 15th anniversary of the Columbine shooting, according to the court complaint.

"This case is a classic example of citizens doing the right thing and calling the police when things seem out of place. By doing the right thing, unimaginable tragedy has been prevented," Waseca police said in a statement.

The teenager, who was arrested on Tuesday, appeared in court on Thursday and was remanded to a secure facility pending his next hearing on May 12, prosecutor Brenda Miller told the Los Angeles Times on Friday.

[Updated, 12:30 p.m. PDT May 2: Officials on Friday filed papers seeking to charge the suspect as an adult, Miller said.]

In the juvenile petition, he is charged with four counts of attempted murder in the first degree, two counts of attempted criminal damage to property in the first degree and six counts of possession of an explosive or incendiary device, Miller said.

The Times does not name juveniles in criminal proceedings unless they are charged as adults.

According to the complaint, the youth intended to carry out his attack on April 20, the anniversary of the attack that killed 12 students and one educator at Columbine High School in Littleton, Colo., in 1999. The youth idolized Eric Harris, who along with Dylan Klebold carried out the Columbine attack then killed themselves, the complaint says.

After he realized that the date fell on Easter Sunday, when school would be closed, he scrapped the plan and was going to launch the attack in the next few weeks, the complaint said.

“I think he had put enough preparation and forethought into this that he was well on his way to carrying it out,” Waseca police Capt. Kris Markeson told reporters at a news conference. "The youth apparently learned how to build a bomb and obtained some of the materials via the Internet," Markeson said.

The youth admitted to having detonated some explosive devices as practice near Hartley Elementary School, the court document said.

The suspect's plan was chilling, police said. He intended to set off the bombs and kill the school officer as soon as he responded. He also allegedly planned to set a fire in downtown Waseca to sidetrack first responders before bombing the school and shooting students. 

The teenager told police that he expected responding SWAT teams to kill him at the school. He said he would have shot at the arresting officers if he’d had a gun, officials said.

The suspect was reportedly a B-student who played guitar. Students at his school told reporters that he was quiet and shy.

“He seemed like a good kid,” said Lucas Hagel, his math partner. “You’d never expect it from him.”