You are correct. It is
incorrect. It's the same problem as in the first question -- the
dreaded comma splice. The quote marks don't change the
grammar; a stronger stop is needed. A period would do.
1. "I see a light at the end of the tunnel," Russial said. "It's probably a mirror."
The following would be OK too:
2. "I see a light at the end of the tunnel; it's probably a mirror," Russial said.