Chad Bungard, deputy staff director and chief counsel at a House Government Reform subcommittee, said he learned of the inquiry from the inspector general's office at the Interior Department, which also is investigating.
"I think they're doing the right thing by pursuing the criminal matter in this case," he said.
FBI spokesman Bryan Sierra declined to comment.
Bungard's subcommittee is holding a hearing on the possible document falsification next week and staffers are preparing to release e-mails from a U.S. Geological Survey hydrologist that suggest the falsification took place.
The e-mails were written from 1998 to 2000 and circulated among a team of scientists studying how water moves through the planned dump site, a key issue in determining whether radiation could escape, and how much.
Scientists at the U.S. Geological Survey validated Energy Department conclusions that water seepage was relatively slow, so radiation would be less likely to escape.
The Energy and Interior departments revealed the existence of the e-mails March 16, and handed them over Tuesday to the House Government Reform subcommittee on the federal work force and agency organization.
The subcommittee, chaired by Rep. Jon Porter (search), R-Nev., plans to make the e-mails public Friday with sensitive information blacked out.
"We don't want to compromise the criminal investigation," Bungard said, adding the agencies themselves were deciding what information to black out.
The Energy Department's inspector general is also investigating the suspected document falsification, and the department is conducting a scientific review as well.
The revelation about the potentially falsified documents was the latest setback to the planned dump 90 miles northwest of Las Vegas — and a victory for Nevada officials who are fighting the project.
Yucca Mountain, approved by Congress in 2002, is planned as the nation's only underground repository for 77,000 tons of defense waste and used reactor fuel from commercial power plants. The material is supposed to be buried for at least 10,000 years beneath the Nevada desert.
But the project has suffered serious problems, including funding shortfalls and an appeals court decision last summer that's forcing a rewrite of radiation exposure limits for the site. The Energy Department recently abandoned a planned 2010 completion date, and department officials have not given a new date.