Two students have been charged with giving a high school's employees marijuana-spiked muffins in a senior prank that sent 18 people to the hospital and triggered an FBI and terrorism investigation.

"I had no idea of the scope of my actions," Ian Walker, 18, said Friday, a day after he and friend Joseph Tellini surrendered to police. They could receive 10 years in prison or more if convicted of felony charges.

Walker is accused of delivering adulterated bran muffins to the teachers lounge of Tellini's suburban school May 16 and claiming they were part of an Eagle Scout project. When Lake Highlands High School employees ate the muffins, they began complaining of nausea, lightheadedness and headaches.

Most of those sickened were quickly treated and released, but Rita Greenfield, an 86-year-old receptionist, spent two days in the hospital.

"They were just thinking it would be fun to get these teachers all silly and giggly," Greenfield said. "I do not think of this as a prank at all. It has caused heartaches and hard feelings."

The FBI investigated because the case involved a contamination of the food supply at a school. A joint terrorism task force found that terrorism was not involved, but determined the muffins contained marijuana and turned up a surveillance video of the delivery.

Walker, an honors student at a nearby Catholic school, and Tellini, 18, were each charged with five felony counts of assault on a public servant, each of which carries penalties of two to 10 years in prison. Because of the presence of illegal drugs, Dallas County prosecutors are seeking an upgrade to the charges that could raise the maximum sentence to 20 years, district attorney spokeswoman Rachel Raya said.

The teens were released on $7,500 bail each. Calls to Tellini's home and attorney were not immediately returned Friday.

Walker and his family spoke Friday in an apology-filled news conference at his lawyer's house.

"It was juvenile and stupid, and believe it or not, I thought I was only participating in a senior prank," Walker said. He won't be allowed to graduate with his class at Bishop Lynch High School because of the incident, his family said.

Ian Walker's mother, Caroline, said she first heard about the tainted muffins while watching the news with her son.

"My first thought when I saw it, because I am a mom, was, 'My heart goes out to the mother of that stupid kid,"' she said.