More than 3 1/2 months after a fatal, three-alarm fire at a downtown Denver construction site, fire investigators have a long list of what did not cause it. But they are still trying to determine what ignited the March 7 inferno, which killed two people and destroyed what was to become a five-story, 84-unit apartment building at Emerson Street and East 18th Avenue, in the North Capitol Hill neighborhood. “Although some possible causes for the fire have been ruled out, it currently is listed as undetermined,” said Greg Pixley, a Denver Fire Department spokesman.