DENVER, Feb. 24, 2011 -- On the same day federal authorities announced the arrest of a Saudi Arabian man in Texas on charges of attempted use of a weapon of mass destruction, ABC News today obtained a letter sent by the FBI to businesses across the country, warning them to watch out for anyone who might be planning a terrorist attack using fertilizer, chemical or pesticide-based explosives.
The FBI's Denver field office has sent a letter to farm supply stores in its region warning employees to be aware of anyone who might be acting in a way that could indicate potential terrorist activity.
"Current trends in terrorist bombings show that fertilizer-based explosives are continuing as a threat throughout the world," the letter states. "Ammonium nitrate and urea-based fertilizers pose an explosive threat if prepared and initiated properly. In addition, certain pesticides can be used to cause widespread harm to people."
FBI officials insist there is no new intelligence suggesting an imminent terrorist attack, saying the letter is a continuing effort of a nationwide program known as "Tripwire"— designed to keep a dialog open between businesses and federal, state and local law enforcement. FBI field offices around the country were told by headquarters to reach out to businesses in their divisions.
"This is our reengagement of those companies to make sure that they're still on alert and that these things still exist," said FBI spokesman Dave Joly.
Word of the warning letters comes just a day after the Wednesday arrest of 20-year-old Saudi Khalid Ali-M Aldawsari in Texas.
Federal authorities allege Aldawsari purchased chemicals and equipment necessary to build an improvised explosive device, and that he also researched potential targets including hydroelectric dams and nuclear power plants.
According to an affidavit, a chemical supplier tipped off the FBI to Aldawsari's attempted suspicious purchase of chemicals that can be used to make an explosive known as trinitrophenol, or T.N.P., or picric acid.
FBI Warns of Fertilizer Purchases for Explosives
A flyer distributed with the FBI letter points out more than a dozen suspicious activities that should raise flags, including anyone "not from the local area" interested in "purchasing large quantities of pesticides, combustibles, of fertilizers containing ammonium nitrate out of season or with cash."
The FBI advises farm suppliers to require valid identification, ask questions and observe responses from all new customers and to contact police if something seems out of place.
"The timely reporting of suspicious activity, which is a partnership between the business community and law enforcement, is essential to the success of anti-terrorism efforts," the letter reads.
Authorities have good reason to be concerned with fertilizer-based explosives. In one of the most horrific cases of domestic terrorism, Americans Timothy McVeigh and Terry Nichols were convicted of detonating a bomb made with large quantities of ammonium nitrate fertilizer outside a federal building in Oklahoma City in 1995, killing 168 people.
Since then, ammonium nitrate fertilizer has become so heavily regulated that many farm supply stores have stopped selling it.
"We just quit selling it because of the problems associated with it, the regulatory requirements, the tracking," said Wayne Gustafson, vice president of agronomy with AgLand Inc., an Eaton, Colorado-based farm supply company. "We just switched to other products that have less exposure."
AgLand was one of many Colorado farm suppliers to receive the warning. Gustafson said he is used to hearing from officials from the Department of Homeland Security and said it was "slightly unusual" get the letter from the FBI.
"It kind of gets your attention," he said. "It's just a good reminder even though we are on our toes all the time anyway."