WASHINGTON -- Investigators failed to turn up conclusive evidence of criminal actions in Yucca Mountain e-mails that implied quality assurance documents might have been falsified, according to a report released Thursday.

But the e-mails suggested management shortcomings on the nuclear waste project allowed for a "poor attitude" to fester among a hydrologist and his supervisor who were tied to many of the messages, the report stated.

A 40-page document issued by the Interior Department inspector general sheds light on the motivations of U.S. Geological Survey employees whose e-mails triggered upheaval within the Yucca program when they were disclosed in March 2005. The report was a redacted version of material forwarded to federal prosecutors in Nevada in December following an investigation by the Interior Department and the Department of Energy.

Daniel Bogden, U.S. Attorney in Nevada, declined to pursue criminal charges. Officials said prosecutors concluded the activity failed to reach a criminal level.

"The investigation revealed the substance of several questionable e-mails, and the related conduct discussed, either did not occur or could not be substantiated," inspectors said in a summary of their work.

Investigators said they parsed 19 e-mails that were originally disclosed, plus another seven that were uncovered later. They interviewed authors and recipients, and discussed the messages with experts who were brought in by the Department of Energy to assess their impact on repository science.

DOE's review concluded the work was technically sound but was not suitable to be used in a formal repository license application and is being replaced.

Most of the e-mails were written between 1998 and 2000 by Joseph Hevesi, a USGS research hydrologist working at the Yucca site, and were addressed to Alan Flint, his supervisor in Sacramento, Calif.

Investigators said Hevesi and Flint "had limited managerial oversight" from DOE and USGS officials as they developed a computer model of how water might infiltrate Yucca Mountain toward the repository level, where nuclear waste would be stored.

Excerpts from the e-mails rocked the Yucca program when they were made public. In one message discussing a research exercise, the author, later identified as Hevesi, wrote, "Wait till they figure out that nothing I've provided them is QA," referring to quality assurance. "If they really want the stuff they'll have to pay to do it right."

Based on their questioning of Hevesi, investigators said that e-mail and others conveyed his frustration with quality assurance rules that require meticulous documentation.

Testifying last summer before a congressional subcommittee, Hevesi said his e-mails were poorly worded and "water cooler talk" and he did not falsify information.

In their report Thursday, inspectors said Hevesi's frustration was fueled by Flint's "lack of supervisory oversight."

An official with Sandia National Laboratories who did a technical review of the e-mails told inspectors they "provided insight into Hevesi's and Alan Flint's poor attitude and demeanor," the report stated.

"According to the (Sandia) official, Alan Flint and Hevesi isolated themselves from the rest of the project and made it known they did not want to work with anyone else except each other," the auditors said.

Flint and Hevesi remain employed at the agency, but are no longer involved in Yucca work.

Sen. Harry Reid, D-Nev., expects to seek more details about the probe, spokeswoman Sharyn Stein said.

"The summary released today does not show the kind of thorough investigation we had hoped for," Stein said.

Rep. Jon Porter, R-Nev., chairman of a House subcommittee investigating the e-mails, said the report sheds light on "a culture of mismanagement at DOE" no matter the lack of criminal prosecution.

Rep. Shelley Berkley, D-Nev., said, "What it really confirms is that the quality assurance program at Yucca Mountain is totally ineffective. But until we learn more about why no criminal charges have been filed as a result of this e-mail scandal, the story remains incomplete."