Updated

Denver police say two 15-year-olds put their lives at risk when they broke into an empty school building carrying BB guns that appeared to be long rifles.

Department spokesman Sonny Jackson said Tuesday the guns were found after a three-hour standoff during which police tried to coax the teens out using the school intercom system. Police said the weapons looked like assault rifles.

Two backpacks were also recovered containing items the teens were apparently attempting to steal. Police were worried they might have contained explosives and they brought in a SWAT team and the bomb squad, which sent a robot into KIPP Montbello College Prep Middle School.

Jackson said the standoff could have ended tragically.

"A lot of BB guns look realistic. We don't want anyone to get hurt," he said after the ordeal ended. He said the two students who were arrested did not attend that school.

The teens were in custody Tuesday and it will be up to the Denver District Attorney's Office to determine if they will be charged as adults.

Neighbors were called and told to stay indoors. The school reopened after police searched the building.

Police were called after a cleaning crew spotted the teens breaking into the school. Jackson said the youths may have tried unsuccessfully to break into another school an hour before the standoff began.