Nico can sense that Reiner is not really dead once they retreated from St. Helens and he told the rest but they won't believe him cause 1 he is still a Kid in this and 2 they still don't know he is a Child of Hades yet (depends if you already revealed it in 4 or 5) and 3 that bolt is so Powerful nothing can be alive in that so they believe that Reiner is very dead the people that can Believe him is Lee, will, and Possibly Bianca but with her in the Hunters I think she can be a little help