In the months leading up to the Decimation, the Office of National Emergency created a new series of Sentinels, manufactured by Stark Industries as armored battle mechs with living pilots instead of artificial intelligence. After M-Day, they were pressed into service as watchdogs for the country's surviving mutant population at Xavier's Mansion and the 198 camp. The original model Sentinel Squad were replaced by upgraded versions during Civil War: X-Men.
During the Messiah CompleX crossover, the Sentinel Squad pilots were infected by nano-sentinel technology, depriving them of free will and turning them against the mutant population. The Sentinels were destroyed by the X-Men, but some of the pilots survived as new, bio-metallic entities who fled and haven't been seen since.