WASHINGTON (AP) -- Terrorists may seek to down aircraft by shining powerful lasers into cockpits to blind pilots during landing approaches, U.S. officials warned in a bulletin distributed nationwide.

The memo, sent by the FBI and the Homeland Security Department, says there is evidence that terrorists have explored using lasers as weapons.

There is no specific intelligence indicating al Qaeda or other groups might use lasers in the United States, they added.

In September a pilot for Delta Air Lines reported an eye injury from a laser beam shone into the cockpit during a landing approach in Salt Lake City, Utah. The incident occurred about 5 miles (9 kilometers) from the airport. The plane landed safely.

FBI and other federal officials are investigating. It is not clear if a crime was committed or if the laser was directed into the cockpit by accident.

Steve Luckey, a retired airline pilot who is chairman of the Air Line Pilots Association's national security committee, said pilots are concerned about a recent increase in laser incidents, but do not know what to make of them. He said he has learned of two or three cases in the past 90 days.

"The most recent incidents appear to be aimed at pilots in the vicinity of airports," Luckey said. "A few seem to be intentional, and we're wondering why and what's going on."