Illinois high school student Thomas Braasch put up a chilling message on Facebook last week: he was going to bring a gun to school and kill his 16-year-old schoolmate the next day, according to the Cook County Sheriff's Office.

Now Braasch, 19, is behind bars today, charged with disorderly conduct for threatening the life of his classmate, although authorities say he did not have a gun with him when he was apprehended in school.

Braasch posted the Facebook message on Wednesday, police said. By that evening, at least five alarmed parents who were alerted by their children of the threat called the school to alert school administrators that an attack might occur, authorities said.

"We felt like we were at an advantage by 6 o'clock on Wednesday," John Byrne, superintendent of the Alan B. Shepard High School in Palos Heights, told ABC News today. "Because it was on Facebook, it got kind of publicized. That was the best thing. Sometimes Facebook gets criticized, but on the other hand, when something like this happens, it's useful."

By the time Braasch arrived at school Thursday morning on the school bus, authorities were ready to escort him to an administrative office and start reviewing the situation, said Byrne. They did not find any weapon on him, nor did they find a weapon when they did a sweep of the school. Detectives took Braasch into custody in the afternoon, charging him with disorderly conduct, which is a class 4 felony.

According to Byrne, Braasch was a student with some academic problems, but he did not raise any red flags. Braasch and the 16-year-old student had allegedly been bickering since last spring.

"It never came to blows or anything, at least, until this incident," Byrne said.

Braasch's bond was set at $150,000 on Sunday, with his next court date on Tuesday. ABC News could not reach Braasch for comment.