According to an opinion piece by former NASA scientist Gilbert Levin, NASA found evidence of alien life in the 1970s but ignored it. As per Levin, NASA sent two Viking orbiter and lander pairs to the planet to conduct experiments - results of which seemed to indicate that something was happening on the surface. Levin says data curves signaled the detection of microbial respiration.

Mars warning: Life may exist in Red Planet caves - but it is NOT ‘place to escape Earth’
https://www.express.co.uk/news/science/1190659/mars-life-earth-exomars-esa-red-planet-habitable-latest-news-update

Former NASA Scientist Is 'Convinced' We've Already Found Evidence of Life on Mars
https://www.sciencealert.com/former-nasa-scientist-is-convinced-we-ve-already-found-evidence-of-life-on-mars

Nasa found evidence of life on Mars in the 1970s, former scientist says
https://www.independent.co.uk/life-style/gadgets-and-tech/news/nasa-alien-life-proof-mars-space-viking-lr-a9156001.html

Former NASA scientist says they found life on Mars in the 1970s
https://edition.cnn.com/2019/10/15/us/nasa-life-on-mars-intl-hnk-scli/index.html