Police and parents in Denver are asking why three middle-school-aged boys brought guns and a smoke device to their school Friday morning. All three boys, whose identities have not been revealed because they are minors, were arrested Friday and remained in custody Saturday.

The guns, which were loaded, were not discharged and no one was hurt. The first people to notice the three boys had weapons were their classmates, who told teachers.

"Our students are to be commended for seeing something and saying something right away," Denver police Commander Paul Pazen told reporters.

Skinner Middle School went into a full lockdown that began at 8:40 a.m. local time and ended at 9:13 a.m., after the boys had been arrested and the weapons secured. In accordance with rules laid out in the Denver Public Schools’ Emergency Response and Crisis Management classroom guide, the other students were herded into auditoriums, science labs and classroom doors were locked. The procedure, with which both students and faculty are familiar, is practiced as part of mandatory drills every semester.

"Unfortunately, in today's world, we have to train for this in our schools," Denver schools security chief Michael Eaton said at a news conference. Colorado was one of many states to institute lockdown drills after two teenagers murdered 12 students and one teacher before killing themselves at Columbine High School in Lakewood in 1999. There has been at least one lockdown instituted in each of the past six years.

Colorado also was the setting for a high school shooting in Centennial in 2013, in which one student died eight days after being wounded by a gunman who killed himself at the school, and a highly publicized shooting at a movie theater in Aurora in 2012, in which 12 people were killed and 70 people were injured.

At 9:15 a.m., with the students apprehended and the weapons confiscated, the school day resumed, though many parents came to school and took their children home. A number of Skinner parents were upset that they were not notified immediately after the school went into lockdown mode, though protocol states that faculty should not communicate with anyone besides school administrators during a lockdown.

A student quoted in a report from a local television station, KMGH-TV, suggested the boys’ plans involved an unnamed teacher.