The U.S. Air Force is investigating a racist incident on the campus of the U.S. Air Force Academy Preparatory School in Colorado Springs, Colorado.

Slurs were discovered on the doors of five African-American U.S. Air Force Academy Prep School cadets’ rooms earlier this week. The U.S. Air Force says it is investigating the incident. A Pentagon official says the “n-word” was discovered on the doors Monday. The incident occurred one day following protests across NFL stadiums where some football players took a knee during the national anthem.

“The Academy’s Security Forces are looking into the matter. We’re unable to release any additional information at this time due to the ongoing investigation,” said Lt. Col. Allen Herritage, an Air Force Academy spokesman in a statement to Fox News.

The U.S. Air Force Academy Prep School is located on the same campus as the Air Force Academy in Colorado Springs, Colorado. The prep school is a one-year program for about 240 cadets who spend a year preparing to enter the U.S. Air Force Academy.

The Air Force Academy’s top officer said racism was a “red line” at the school, which produces nearly 1,000 graduates a year to serve as officers in the U.S. Air Force and vowed “significant repercussions.”

“It’s not who we are, nor will we tolerate it in any shape or fashion,” said Lt. Gen. Jay Silveria, superintendent of the U.S. Air Force Academy.

“The Air Force Academy strives to create a climate of dignity and respect for all — period — those who don’t understand that are behind the power curve and better catch up,” he added.