The U.S. attorney's office will not pursue criminal charges over allegations of paperwork fraud by government scientists on the Yucca Mountain nuclear waste dump in Nevada, the Energy Department's inspector general announced.

In a memo released Tuesday, Inspector General Gregory H. Friedman said he concluded his criminal investigation in December and turned the results over to the U.S. attorney's office for the District of Nevada. The U.S. attorney declined to pursue criminal prosecution, the memo said.

"Nonetheless, the actions of those involved — which have been described by observers as irresponsible and reckless — have had the effect of undermining public confidence in the quality of the science associated with the Yucca Mountain Project," the memo said.

Testifying at a congressional hearing Tuesday, Friedman said prosecutors had told him they "could not show intent and the actions did not rise to the level of criminality."

At issue were e-mails exchanged among U.S. Geological Survey hydrologists between 1998 and 2000 that suggested they were falsifying documentation of their work to satisfy quality assurance standards.

Yucca Mountain is planned as the first national repository for nuclear waste and is meant to hold at least 77,000 tons of the material. Political opposition, money shortages and other problems — including the e-mail controversy — have delayed the project, and Energy Department managers now can't say when the site will open.

The Energy Department revealed the existence of the e-mails a year ago. Portions of the e-mails that were made public indicated that scientists made up dates, deleted inconvenient data and kept one set of documents for themselves and another for quality-assurance officials.

"This is as good as it's going to get. If they need more proof, I will be happy to make up more stuff," one scientist wrote.

A scientific review by the Energy Department concluded in February that the scientists' work was sound, but it is being redone anyway — at cost of millions of dollars — because quality-assurance requirements were flouted.

The scientists were studying how water moves through the dump site 90 miles northwest of Las Vegas, a key factor in how much radiation can be released.

Despite the decision not to prosecute, Friedman was sharply critical Tuesday of Energy Department internal controls, particularly the six-year delay from when the e-mails were written to when they became known to top Energy Department managers.

"We could not find a satisfactory explanation," his report said, noting that at least one supervisor was aware of the e-mails around when they were written.

Bechtel SAIC contractors found the e-mails in November 2004 while conducting reviews required for DOE to apply for a Nuclear Regulatory Commission license. Even after that it took another four months for top managers to see them.

That delay was partly due to "the disruption of work during Bechtel's holiday season shutdown," according to an internal DOE report quoted by the inspector general.

Chairing a hearing Tuesday of a House Government Reform subcommittee, Nevada Republican Rep. Jon Porter said Yucca is "consistently failing." But he said the decision on whether to prosecute "is for the U.S. attorney's office to determine."