TAMPA, Fla. — Police were already keeping an eye on 17-year-old expelled student Jared Cano when they were tipped off that he was allegedly planning to bomb his old high school when classes resumed. In his apartment, they found shrapnel, plastic tubing, timing and fuse devices that he was amassing in a plot he intended to be worse than the Columbine mass killings, police said Wednesday.

Before Tuesday's discovery, Cano has been arrested several times, most recently accused of breaking into a house and stealing a handgun, Tampa police said. He had a court-ordered curfew and was on a police watch list. Officers checked up on him from time to time.

"We've been very, very familiar with him," police Maj. John Newman said.

Besides the bomb-making materials, officers said they also found a journal with schematic drawings of rooms inside Freedom High School and statements about Cano's intent to kill specific administrators and any students who happened to be nearby next week.

His juvenile arrests included burglary, carrying a concealed weapon, altering serial numbers on a firearm and drug possession. None had stuck. They had been either dismissed or no action had been taken, beyond putting his name on the police watch list.

The school scheme was mapped out minute-by-minute and he wanted to cause more casualties than the 1999 Columbine High School massacre, which killed 13 before the two shooters killed themselves, said Police Chief Jane Castor.

Police and the school system "were probably able to thwart a potentially catastrophic event, the likes of which the city of Tampa has not seen, and hopefully never will," Castor said.

He also had a marijuana-growing operation, police said. On his Facebook page, he says he attends the "University of Marijuana," where he is studying "how to grow weed."

Principal Chris Farkas and other administrators knew Cano, too. He'd been expelled in April 2010. Farkas said Tuesday that Cano likely would have been "red-flagged" as soon as he stepped on campus and probably would not have been able to pull off his plan.

Farkas said he is accustomed to all sorts of threats at a school of 2,100 on a large campus in the northern suburbs. Still, he was spooked about what might have been.

"My first response was shock," he said. "I wanted to see if it was a real threat."

"Once I found out and saw the information and saw what was taken from the apartment complex, that was when the reality and the fear set in that this was a real situation," he said.

The St. Petersburg Times reported that prosecutors at a hearing Wednesday said that when Cano was arrested he repeated his plan to detonate a bomb and cause mass casualties at Freedom, which opened in 2002 and was named to honor the victims of 9/11.

Cano tried to speak when he appeared before a judge but was quickly hushed by a public defender standing beside him.

"The plot wasn't..." Cano said, before the public defender stopped him and told the judge that "he has no comment," according to the Times.

Police told Farkas that Cano worked alone. Parents of every student got a recorded call informing them about Cano's arrest, said the principal of the high-performing school built to handle the overflowing northern suburbs in an area some locals refer to as New Tampa.

Authorities did not name the administrators targeted nor would they disclose who tipped them off.

After Cano was expelled from Freedom, he attended a charter school and left voluntarily in March, according to Hillsborough County schools spokeswoman Linda Cobbe. At that point he was 16 and could have chosen to drop out. He was not registered to attend classes this upcoming school year.

Cano faces felony charges of possessing bomb-making materials, cultivating marijuana, possession of drug paraphernalia, possessing of marijuana and threatening to throw, project, place or discharge a destructive device. He was being held in a juvenile lockup in Tampa.

The Department of Children and Families said the agency investigated Cano's family in 2009. A spokesman gave no other details except to say DCF found no evidence of abuse or neglect.

Police said his mother let them search the apartment in a modest complex just a few miles from the school. The Tampa Tribune reported that Cano's great-grandfather Elliot Horning said that Cano's mother, Michelle, was divorced from his dad and worked as a math teacher at another Tampa high school. His mother was not at the apartment Wednesday.

Cano's Facebook page includes photos of him holding a machete and drinking from a bottle of malt liquor.

He lists two favorite quotes: "lessons not learned in blood are soon forgotten" and "dont trust anybody, cuz they all just wait for you to s--- a brick of gold so they can take it." He listed just 25 friends and no one that was out around the apartment complex seemed to know him.

On his Facebook page Tuesday morning, Cano wrote: "i jut did the dumbest thing ever!"

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Associated Press writers Christine Armario and Kelli Kennedy contributed from Miami.