Russillo & Kanell share their thoughts on the reasons they believe the NFL's television ratings are declining this season. (1:43)

Speculation about plummeting ratings for NFL games led the league to circulate an internal memo Thursday, saying the decline was a result of a variety of factors.

Through the fourth week of the season, ratings across all networks are down 11 percent.

"Prime-time windows have clearly been affected the most, while declines during the Sunday afternoon window are more modest," the NFL's top media executives, Brian Rolapp and Howard Katz, wrote to the league's media committee in a memo obtained by ESPN. "While our partners, like us, would have liked to see higher ratings, they remain confident in the NFL and unconcerned about a long-term issue."

Over the last 15 years, NFL viewership overall, the memo said, has increased 27 percent amidst a prime-time viewership fall of 36 percent.

One of the factors, the executives said, was the presidential election, something that they noted was not unprecedented.

"In 2000, during the campaign between George W. Bush and Al Gore, all four NFL broadcast partners suffered year-over-year declines -- Fox was down 4 percent, CBS was down 10 percent, ABC was down 7 percent and ESPN was down 11 percent," the memo said.

The executives also said that they saw "no evidence that concern over player protests during the national anthem is having any material impact on our ratings. In fact, our own data shows that perception of the NFL and its players is actually up in 2016."