Washington (CNN) -- The Department of Homeland Security and the FBI have warned police across the United States that al Qaeda has a "continuing interest" in attacking oil and natural gas targets, a department spokesman said Friday.
The warning issued Thursday came as a result of information seized during the May 2 raid that killed Osama bin Laden in Pakistan, a U.S. official said.
"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.
"However, in 2010 there was continuing interest by members of al Qaeda in targeting oil tankers and commercial oil infrastructure at sea."
It is "unclear if any further planning has been conducted" since the middle of last year, Chandler said.
CNN obtained a copy of a DHS/FBI intelligence bulletin that says al Qaeda has been interested in "targeting unspecified oil tankers abroad" as a means to "draw the West into an extreme economic crisis" by disrupting a significant portion of the oil supply for several years.
The advisory says al Qaeda wanted to target oil tankers in the Indian and Atlantic oceans, as well as the Arabian Sea, but the terrorist group "was opposed to targeting tankers in coastal areas with large Muslim populations."
The warning says al Qaeda "believed an effective method for sinking oil tankers was to hijack them and then detonate explosives from the inside." The group thought it would be more difficult to stage an attack from outside a ship, as it could "require several explosive charges since tankers are divided internally into multiple watertight sections."
The bulletin says in 2010 al Qaeda tried to get information about the size, layout and construction of oil tankers to help with attack planning. The document says al Qaeda also recognized the importance of pre-attack surveillance of a possible target and recommended conducting trial runs.
The U.S. official said there's "not a lot of detail" concerning the information, and strikes against the oil infrastructure have "long been part of the al Qaeda playbook."
"No specific attack method was identified in 2010 and there was no reference to a specific date or time of the threatened attack. The global marine industry operates under an international convention intended to prevent terrorist incidents from occurring on commercial ships and in port facilities," the official said.
"If actionable intelligence is received and additional security measures are necessary, requirements or guidance is conveyed through a U.S. Coast Guard-issued Maritime Security Directive and/or Port Security Advisory -- neither have been issued at this time."
The U.S. operation to kill bin Laden in Abbottabad, Pakistan, collected a trove of material from the compound, including 10 hard drives, five computers and more than 100 storage devices such as disks and thumb drives, a senior U.S. official said.
Investigators are poring over the items for intelligence. Not long after the raid, investigators said materials taken from bin Laden's compound revealed details about a possible attack on the 10th anniversary of 9/11.
The material suggests that al Qaeda was particularly interested in striking Washington, New York, Los Angeles and Chicago, according to a law enforcement official who spoke on condition of anonymity.
U.S. authorities have found that al Qaeda appeared especially interested in striking on significant dates such as July Fourth, Christmas and the opening day of the United Nations.
As early as February 2010, al Qaeda members discussed a plan to derail trains in the United States by placing obstructions on tracks over bridges and valleys, an alert said, according to one law enforcement official.
The U.S. official said Friday that "if we discover information that points to an imminent threat, the United States government will, of course, swiftly take appropriate measures."
"As for the oil tanker information that is out there, just like the train information from earlier this month, we encourage our partners and the press not to be surprised that we found ideas for terrorist plotting in the home of the world's most wanted -- and most dangerous -- terrorist."