Osama bin Laden's personal files revealed a brazen idea to hijack oil tankers and blow them up at sea last summer, creating explosions he hoped would rattle the world's economy and send oil prices skyrocketing, the U.S. said Friday.

The newly disclosed plot showed that while bin Laden was always scheming for the next big strike that would kill thousands of Americans, he also believed a relatively simpler attack on the oil industry could create a worldwide panic that would hurt Westerners every time they gassed up their cars.

U.S. officials said the tanker idea, included in documents found in the compound where bin Laden was killed nearly three weeks ago, was little more than an al-Qaida fantasy. But the FBI and the Homeland Security Department issued a confidential warning to police and the energy industry Thursday. The alert, obtained by The Associated Press, said that al-Qaida had sought information on the size and construction of oil tankers, had decided that spring and summer provided the best weather to approach the ships, had determined that blowing them up would be easiest from the inside and believed an explosion would create an "extreme economic crisis."

Bin Laden's documents also revealed that in February 2010, the terror group identified New York, Washington, D.C., Los Angeles and Chicago as important cities that should be attacked; and it eyed specific dates, including the 10th anniversary of the Sept. 11 attacks, Christmas, July Fourth and the State of the Union address, according to a similar alert issued Friday, obtained by the AP. There is no information indicating that there are plots involving these cities, dates and tactics under way.

"We are not aware of indications of any specific or imminent terrorist attack plotting against the oil and natural gas sector overseas or in the United States," Homeland Security spokesman Matthew Chandler said in a statement Friday. "However, in 2010 there was continuing interest by members of al-Qaida in targeting oil tankers and commercial oil infrastructure at sea."