Envelopes containing matches that were rigged to ignite when opened have been received through the mail at the offices of at least 14 state governors in the last two days.

The mailings, under investigation by the F.B.I. and the Department of Homeland Security, bear a return address that names two inmates at a maximum-security prison in Nevada. But a Nevada corrections official said it was unclear whether they were the actual senders.

Aides to several governors, including Mitt Romney of Massachusetts, said they had been told by the Federal Bureau of Investigation that the case was being treated as one of domestic terrorism, and Jennifer Meith, a spokeswoman for the Massachusetts Fire Marshal's Office, said that was her understanding as well.

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The governors who were sent the envelopes are Democrats and Republicans alike. In addition to Mr. Romney, of Massachusetts, and Ms. Martz, of Montana, they are George E. Pataki of New York, Rick Perry of Texas, Dirk Kempthorne of Idaho, Mike Johanns of Nebraska, Gary Locke of Washington, Olene S. Walker of Utah, Bill Owens of Colorado, Theodore R. Kulongoski of Oregon, Dave Freudenthal of Wyoming, Kenny Guinn of Nevada, Linda Lingle of Hawaii and Janet Napolitano of Arizona.