Well, interestingly DiGregorio states the
My book "Mars The Living Planet -should have been released in June 1997, however due to circumstances beyond my control (the publisher) it will be released closer to August. Why should it matter? Chapter 9 is written by Dr. Gilbert V. Levin who concludes absolutely that his experiment on the Viking Landers in 1976 discovered living microorganisms in the soil of Mars. In examining over 100 different lines of evidence myself over a four year period --I concur with Levin's conclusion.
From his description of his book on Amazon listening to him on C2C he was no light-weight
https://www.amazon.co.uk/Mars-Living.../dp/1883319587
Both G.V. Levin and P.A. Straat who designed the Viking experiments agree 'The Case for Extant Life on Mars and its Possible Detection by the Viking Labeled Release Experiment' (Astrobiology, Volume 16, Number 10, pp. 798-810, 2016)
http://www.gillevin.com/Mars/Astrobi...and_Straat.pdf
What's more DiGregorio seems convinced NASA is very well aware that they have had proof since 1976, he claims the later Mars probe didn't really have much decent equipment to look for life. The British one that did never made it to the surface.