The attacks are meant to cause fear, and chlorine gas - which was used as a chemical agent in World War I - could be an attempt to cause more fear, officials said.

U.S. soldiers operating 12 miles northwest of Fallujah discovered a car bomb factory Feb. 20, Odierno said. They found numerous artillery rounds, mortar rounds, bombs, rockets, gutted anti-aircraft shells, a pickup truck and three other vehicles that were already in various stages of preparations as car bombs.

"We also found ingredients to be used to devise or enhance explosives such as fertilizer and chlorine cylinders," the general said.

The introduction of chlorine illustrates that the enemy continues to alter its tactics, Odierno said.

"What they're trying to do is try to adapt in such ways where they can continue to create instability, and that's what they're doing, especially with these chlorine (vehicle-borne improvised explosive devices)," he said.

But as the enemy changes its tactics, he noted, so will coalition forces.

The general said eight coalition helicopters have been shot down since Jan. 20. A UH-60 Black Hawk came down yesterday north of Baghdad. The cause of that downing is still being investigated, Odierno said, but initial reports indicate enemy fire brought it down.

"We are aggressively examining the conditions of each incident and adapting tactics and techniques to address the issue."

The helicopters may have run into enemy ambush sites, Odierno said.

"We are studying those intently, and we're trying to learn from those, and we will learn from those and we will adapt our tactics," he said. "I think they've probably been trying to do this for a long time, but my guess is we have a cell out there that's somewhat effective."

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