BOULDER, Colo. -- A 10-year-old boy is the third schoolboy suspected of trying to harass and sexually assault a girl based on her race.

Boulder police said two 13-year-old boys made threatening phone calls on Monday to the cell phone of an Asian-American schoolmate.

They were arrested Thursday on suspicion of using sexually explicit and violent words to threaten to rape the girl, 12.

"They left a variety of messages. I know they had one conversation with the girl, and they left two voice-mail messages," said Sarah Huntley, Boulder police spokeswoman. “The patrol and school officers involved in this case are very concerned about the level of threat, the level of language, the violence of the language and they felt it was important to take action."

Friday, police learned the age of the third suspect, 10, allows them to charge him as well, bringing the third arrest in the case.

All are being processed through a juvenile detention center.

Police said the suspects also threatened to kill the 12-year-old, and investigators will recommend the bias-motivated crime charge because they believe that was the underlying reason for the alleged harassment.

Police and a school official would not identify the middle school or the elementary school involved due to federal privacy laws.

“We don't tolerate bullying in any shape or form,” said Briggs Gamblin, spokesman for Boulder Valley schools. "We put a lot of effort, time and money into making school a safe, welcoming and nurturing place for each and every one of our kids. We think our programs are working very well but always look for ways to continually improve them."