Two federal agencies launched investigations Wednesday into evidence that government scientists had submitted phony data to help prove that a proposed nuclear dump at Yucca Mountain in Nevada would be safe.

The disclosure could delay the long-troubled project and undermine assurances that the waste dump would pose no harm to the public for thousands of years.

But Energy Department officials cautioned Wednesday that even if some data were falsified, it would not necessarily discredit all of the research.

Energy Department lawyers discovered a series of e-mail exchanges between scientists that discussed fabricating documentation for a key scientific study about ground-water penetration into Yucca Mountain.

The study was conducted by the U.S. Geological Survey, a part of the Interior Department. It concluded that the deep tunnels intended to hold radioactive waste inside Yucca Mountain would remain dry for thousands of years, and that radiation could not quickly leak into the ground water.

That scientific question is one of the most critical surrounding Yucca Mountain, a complex engineering project that is running 14 years behind schedule and could end up costing $100 billion. The mountain is supposed to safely isolate radioactive materials for hundreds of thousands of years.

Wednesday’s disclosure inflamed opponents in Nevada, who long have said that the federal government rigged its scientific research to get the dump licensed as soon as possible.

“This proves once again that DOE must cheat and lie in order to make Yucca Mountain look safe,” said Senate Minority Leader Harry Reid, D-Nev.