In the summer of 2017, a U.S. Congressman asked a select NASA panel about the possibility of a civilization having existed on Mars "thousands of years ago" - to which a NASA scientist replied, "there is no evidence" and it was "extremely unlikely."
The exact question from U.S. Congressman Dana Rohrabacher to Dr. Kenneth Farley, the Project Scientist for the Mars 2020 rover mission, was:
"You've indicated that Mars was totally different thousands of years ago. Is it possible that there was a civilization on Mars thousands of years ago?" To which the NASA scientist replied, first with a correction:
"The evidence is that Mars was different billions of years ago. Not thousands of years ago", after which he then continued:
"There is no evidence that I'm aware of". The Californian politician however pressed him further:
"Would you rule that out?" To which Farley answered with a humorously sardonic dismissal:
"I would say that is extremely unlikely."
This appears to be the first documented public statement from NASA regarding the possibility that an intelligent race may have once inhabited Mars. A somewhat cagey reply that does not explicitly rule it out, but avoids giving a direct answer nonetheless.
Indeed, we should wonder why NASA scientists are so arrogantly dismissive of the idea that an ancient civilization could have existed on Mars. Especially when cosmologists and astronomers are now seriously considering the possibility, even probability, that thousands of years ago, ancient space-faring civilizations may have already travelled to, explored and colonized our solar system and others in our galactic neighbourhood.
One astronomer has written a paper that suggests we should specifically look for artifacts of these possible explorers on some planets and moons in our solar system, especially Mars, and there may already be ample evidence of such artifacts and structures having been found as revealed in images taken by NASA rovers and orbiters. 
We should also not forget NASA's own report commissioned from the Brookings institute in 1959,  when it was advised of the possibility that artifacts from ancient explorers to our solar system may well be found on Mars by NASA space missions.
So who is behind the times here? Why is this space agency not taking seriously the implications of their own science programmes such as the Kepler Mission's exo-planet data that revealing the likelihood of millions of Earth-like planets existing in our galaxy?
With so many potential Earth-like planets, the possibility of countless civilizations existing is inescapable. And that some of them may have reached out to investigate our own solar system, is for sure in the minds of NASA and everyone else involved with space exploration.
Only perhaps, they are not sharing their thoughts with the world on this matter. Or they are not allowed to.
M J Craig, Mars Archaeology Archive
Author of "Secret Mars"
1. Paper: Prior Indigenous Technological Species, Jason T. Wright
2. Webpage: Potential Archaeological Evidence on Mars, Mars Archaeology Archive https://www.secretmars.com/index-by-maa-ref
3. Article: Its Official NASA Told NOT to Discuss Alien Life, Secretmars.com