Homemade, Cheap and Dangerous

Stephen Swain, former head of the international counterterrorism unit at Scotland Yard in London, said the culprits in each case had been trained by seasoned al-Qaeda operatives "without a shadow of a doubt."

"It's a common thread throughout the entire theater of al-Qaeda-style operations," said Swain, who retired last year and is a senior consultant for Control Risks Group, an international security firm. "There's quite a lot of training going on in this regard."

Simple Recipes

Terrorist groups have been using homemade explosives for years. In February 1993, Islamic radicals drove a truck loaded with about 1,500 pounds of urea nitrate -- a fertilizer-based explosive -- and hydrogen-gas cylinders into a garage underneath the World Trade Center.

The bomb killed six people and injured more than 1,000. Investigators determined that the cell built the bomb in New Jersey by consulting manuals brought from Pakistan. Two years later, the ringleader of the plot, Ramzi Yousef, tried but failed to use homemade liquid explosives to down 11 airliners crossing the Pacific Ocean.

The same year, Timothy J. McVeigh blew up the Alfred P. Murrah Federal Building in Oklahoma City with a truck bomb consisting of 5,000 pounds of fuel oil and fertilizer, killing 168 people.

Since then, it has become more difficult to purchase large quantities of fertilizer without attracting attention; sellers in the United States and many European countries are supposed to notify authorities of suspicious customers.

The last known attempt by al-Qaeda to construct a fertilizer-based bomb in Europe came in 2003, when a cell operative bought 1,200 pounds of bagged Kemira GrowHow fertilizer for about $200 from an agricultural supply store in Britain. The salesman, John Stone, later testified that he thought the transaction was strange because it was the wrong time of year to apply the substance and the buyer claimed to have only a small garden plot.

"I hope you're not going around bombing anything," Stone said he told the buyer half-jokingly, although the salesclerk did not notify police.

British authorities were tipped off soon after, however, by an employee at a rental-storage center where the cell was keeping the fertilizer. In April, five members of the group were convicted in the plot, known as Operation Crevice. Many European fertilizer manufacturers have since reduced the amount of ammonium nitrate, a key bomb ingredient, in their products.

Partly as a result, European and U.S. counterterrorism officials said terrorist cells are increasingly turning to peroxide-based explosives, which can be made in much smaller quantities from materials available at drugstores.

The most commonly used compound is triacetone triperoxide, or TATP. Primary ingredients for a homemade batch typically include acetone, which can be found in nail-polish remover, and hydrogen peroxide, a chemical used in hair-bleaching products.

TATP wields about 85 percent of the explosive power of TNT and can be made in a kitchen or bathroom. A dime-size amount of the explosive can ignite a fireball the size of a basketball.