Federal and state investigators questioned Nevada prison inmates Friday after at least 15 governors received letters rigged to catch fire when the envelope was opened.

The letters apparently did not contain writings but bore a return address from Nevada's maximum-security Ely State Prison (search). In three cases, a match inside the envelope flared when the letter was opened, but no one was hurt.

The other letters were intercepted Thursday and Friday during screening or because of an alert issued by the Homeland Security Department (search). In addition to the governors of Nevada and at least 14 other states, Nevada's corrections director received a booby-trapped letter.

The letters listed one or the other of two Ely inmates as the sender, but authorities are not sure if either prisoner was involved, said Glen Whorton, assistant director for the Nevada Corrections Department.

"We're not assuming the names on the envelopes are simply the end of the matter," he said. "Investigators are not just talking to the two inmates."

The Montana Capitol was partly evacuated Thursday when the match burned the letter opened there, but there was no further damage. The letter sent to the office of Nevada Corrections Director Jackie Crawford contained blank paper and a match, which ignited as the paper was pulled out.

Whorton said there was nothing unusual-looking about the envelope. He said letters leaving Nevada prisons are not opened unless there is something out of the ordinary, such as a bulky envelope or an inadequate return address.

Letters were sent to governors in Nevada, Montana, Hawaii, Nebraska, Colorado, New York, Texas, Massachusetts, Idaho, Wyoming, Washington, Oregon, Utah, Vermont, and Arizona.

Of the 15 governors whose staffs confirmed receiving the suspicious letters by Friday afternoon, 11 are Republicans and four are Democrats.