When NASA sent Viking to Mars in 1976 it detected gas activity in the Martian soil. This activity was similar to microbes giving off gas. NASA continued to look for organic matter which it did not find therefore it was concluded no life existed.
Scientists continued to work on the data and concluded that they had in fact detected life. His report states that the instrument for detecting organic matter was very insensitive and did not match the sensitivity of the instrument that detected gas.
Scientists say it will take a number of probes to Mars with specialised missions before we know the answer to life on Mars. The next probe to launch this summer will be for geological study. http://news.bbc.co.uk/1/hi/sci/tech/2941826.stm