A yearlong investigation by The Associated Press has found roughly 17,000 official reports of sexual assaults by students in K-12 schools over a four-year period in the United States. It’s the most complete tally of such assaults among the nation’s 50 million K-12 students, but academic estimates range sharply higher.

The AP investigation examined sexual assaults reported between the fall of 2011 and the spring of 2015.

Colorado didn’t begin tracking incidents leading to suspension, expulsion or referrals to law enforcement until the following 2015-2016 academic year.

But state law enforcement agencies participating in the FBI’s National Incident-Based Reporting System reported 350 incidents of student sexual assault on school property over the two most recently available years.

There is no national requirement for elementary or high schools to track such assaults.