Crop-dusting planes and pilots have been coming under special scrutiny in the past year: FBI Checking Crop-Dusting Planes, Pilots.

WASHINGTON - The FBI has questioned more than 3,000 pilots and aircraft owners, most of them in the past year, amid persistent concerns that terrorists might use crop-dusting planes to mount a biological or chemical attack, newly released documents show.

The interviews have not produced any arrests, according to a senior law enforcement official who spoke on condition of anonymity, but have resulted in terrorism investigations that are still under way.

The effort, outlined in documents submitted to the commission investigating the Sept. 11 attacks, is more extensive than previously disclosed and underscores how seriously the threat is viewed by U.S. authorities.

The level of overall terrorist threats against the United States remains extremely high, Attorney General John Ashcroft told reporters Thursday. He echoed the assessments of other top Bush administration officials that the summer months present a number of opportunities for attacks, ranging from the Olympics in Greece to the presidential nominating conventions in Boston and New York.

“There is without a doubt in my mind a very serious level of activity in terrorism which concerns me greatly,” Ashcroft said at the Justice Department.