Letters bound for governors' offices and rigged to ignite when opened were intercepted in Virginia and West Virginia, bringing to 18 the number of governors who have received similar letters in the past week.

The latest letters never explicitly threatened Gov. Mark R. Warner (search) of Virginia or Gov. Bob Wise (search) of West Virginia, and neither letter ignited Monday. Both bore a return address from Nevada's maximum-security Ely State Prison (search).

Authorities said they were interviewing inmates at the prison.

The letters — which have targeted 12 Republican governors and six Democrats — listed one or the other of two Ely inmates as the sender, but authorities are not sure if either prisoner was involved, Glen Whorton, assistant director for the Nevada Corrections Department, said last week.

"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."

It does not appear that any of the letters contained any writings, only a blank sheet with a match or match head rigged to ignite when the paper was opened. At least three of the letters opened last week caught fire, but no one has been injured.

Letters were sent to governors' offices last week in Alaska, Nevada, Montana, Hawaii, Nebraska, Colorado, New York, Texas, Massachusetts, Idaho, Wyoming, Washington, Oregon, Utah, Vermont and Arizona. In addition to the governors, Nevada's corrections director received a booby-trapped letter.

Robert A. Fisher, a U.S. postal inspector based in Richmond, said the letters are in standard, white business envelopes. The letters bore Las Vegas postmarks and were hand-stamped. Sending a letter intended to do harm to its recipient is a federal felony punishable by up to 20 years in prison, Fisher said.

The letter addressed to the Executive Chambers in Virginia's capitol was spotted by a letter carrier who delivers mail to Warner's third-floor office suites. It was turned over to the FBI for analysis, said Capt. Mike Jones of the Virginia Capitol Police.

All mail bound for Virginia's executive offices on the third floor of the state Capitol is routed through an offsite receiving facility to protect the governor and his staff. "We've done it for this very reason," said Bill Leighty, Warner's chief of staff.

The letter to Wise, addressed to "WVA Governor," made it to his office in the Capitol before it was intercepted. It was flagged by a worker in his office who recognized the return address.