Someone removed bolts from the base of a high-voltage electrical transmission tower, causing it to fall on a second tower and knock out power to 17,000 customers.

The federal Joint Terrorism Task Force (search) and the FBI (search) were investigating.

"We have not determined the motive of this action," FBI agent Mike Johnson said Monday. "It may be terrorism, it may not be. It's just too early to tell."

Both 80-foot towers fell to the ground Saturday. The bolts had been removed from plates connecting the legs of one tower to its base, Police Chief Thomas Bauer said.

"It does look like it's for the purpose of weakening the structure so it would fall," Bauer said. He said anyone with an ordinary wrench could have removed the two-inch bolts.

The incident near this Milwaukee suburb caused a four-hour blackout for 17,000 customers, including General Mitchell International Airport (search) in Milwaukee. Screening equipment was shut down and flights were delayed as passengers and luggage were screened by hand.

Downed wires from the towers lay across railroad tracks much of Sunday, delaying passenger and freight trains.

American Transmission Co., which owns the towers, announced a $10,000 reward for information leading to the arrest and conviction of whoever was responsible. A company spokeswoman estimated the damage at about $300,000.

Authorities have found some evidence, including bolts, they hope will lead them to the culprit, Bauer said. Destroying an energy facility carries a federal penalty of up to 20 years in prison.

In February, Michael Poulin, 62, of Spokane, Wash., was sentenced to 27 months in prison for removing bolts in the legs of about 20 transmission towers in California, Oregon, Washington and Idaho. He said he wanted to show how vulnerable America is to terrorist attack.