There is a new worry for U.S. and Iraqi troops: insurgents mixing bombs with chemicals, perhaps probing U.S. defenses and looking for new and deadlier ways to kill.
In the last month, there have been three incidents in Iraq of insurgents mixing chlorine with explosives in the apparent hope of using the toxic chemical as a deadly weapon.
U.S. officials say new bombs from the insurgents were expected since the Baghdad security plan has locked down the city into a maze of concrete barriers. Traffic patterns now flow away from soft target areas where pedestrians and shoppers congregate; the new bombings show that insurgents are adapting to the new environment created by the Baghdad security plan.
In his weekly briefing, Maj. Gen. William Caldwell said several times that insurgents are "changing tactics." According to Caldwell, they are seeing more suicide bombers wearing vests, more car bombs and more attacks on the edges of the city, or the "belt around Baghdad," as he called it.
Another tactic insurgents are employing with some degree of success: downing helicopters using a "swarming technique" that uses different weapons systems and training them all on a single helicopter all at once. The military announced that the Blackhawk forced to make a "hard landing" on Tuesday was brought down by insurgents using small arms fire and rocket propelled grenades.
Richard Clarke -- President Bush's former top anti-terrorism adviser -- says of the chlorine attacks, "This is the sort of thing we should have anticipated and is not at all surprising that terrorists are using chemicals now." He says the U.S. military "should have labeled it [chlorine] an explosive rather than a commercial chemical in common use." He added, "It is only surprising that it has taken the terrorists as long as it has to figure this out."
So far, in the three incidents where chlorine was suspected to have been used, the explosions have been more deadly than the chemicals.
The first was near Ramadi on Jan 28. A suicide bomber driving a dump truck plowed into an emergency services compound. The dump truck had a chlorine tank in it. The 16 people who died were all killed as a result of the blast, not the chlorine. It's unknown if the chlorine tank was deliberately placed on the truck.
The second came on Feb. 20 near Taji, north of Baghdad. Nine people were killed by the blast and more than 138 were injured. Some people suffered respiratory problems from the chlorine gas.
The latest incident was on Feb 21. A car bomb blew up near a gas station in southeast Baghdad. Two people were killed by the blast and 30 others injured. Some people complained of breathing difficulties because of a "blue" smoke coming from the car. Chlorine is generally a yellowish-green color. It's still unclear if chlorine or another chemical was involved.
In the three incidents, at least 27 people were killed by the blasts and none by chlorine.
Still, the concern for military officials is evident. Exposure to high levels of chlorine could cause burning of the eyes and skin, rapid breathing, narrowing of the bronchi, wheezing, blue coloring of the skin, accumulation of fluid in the lungs and pain in the lung region. And chlorine delivered in concentrated-enough amounts could kill with only a few breaths.
Chlorine in a war environment is not new. The chemical -- widely used in Iraq for purifying water -- has been used as a weapon by Chechens and Tamil Tigers in Sri Lanka.
Anthony Cordesman, an ABC News Terrorism consultant, says chlorine is not an efficient killer because it disperses too easily, particularly when detonated by a bomb. The shockwave alone will disperse chlorine gas too quickly to cause life-threatening damage. But, says Cordesman, chlorine and chemicals in general could be a "a major weapon in terms of publicity."
Iraq's political and military leaders say insurgents are simply trying to frighten the public by using chemical weapons in the hopes of making Baghdad's central government look weak.
Iraqis are sensitive to chemical attacks. During the Iran-Iraq war, chemical weapons were used and hundreds, if not thousands, of Iranians and Iraqis died hideous deaths.
Currently in Iraq, the trial of "Chemical Ali" continues. He was alleged to have used chemical weapons against Kurds in northern Iraq during the Anfal campaign.
Perhaps of even more concern are indications that insurgents are experimenting with the use of other chemicals and using them in conjuntion not only with bombs but also with rockets.
Chemical, bomb, and rocket-making instructions can be found on insurgent websites and throughout the online world in just about every language. And Ian Day, ABC News' Security Consultant, points out there are now online videos showing insurgents making chemical rockets. The use of chemicals, says Day, is "very amateurish for now, but they could learn how to do this better."
The top U.S. ground commander, Lt. Gen. Ray Odierno, says that chlorine canisters and other chemicals have been discovered at several sites in Iraq during raids and for now, they are studying what these new weapons will mean on the battlefield.
The big concern may eventually be the hardest to resolve. Despite the fact that their current bombs are simplistic in design, it's possible that if the insurgents keep testing, they might find a better way. For now, says Odierno, U.S. forces have to do "what we can do to try to stop them from detonating them at all."