Accusations that Iranian aid to Iraq is a danger to American troops there is based on past (2005-2011) actions by Iran against U.S. troops there. Much has been made of how Iran aided Islamic terrorists in Iraq after 2005, especially those who were trying to kill American troops. Iran is not known to be doing that now, but the fact that they did it before and are still calling for the destruction of the United States has a lot of Americans uneasy.
Because of some uniquely Iranian weapons it was possible to trace a specific number of American deaths in Iraq to Iran. Since 2014 the number of such deaths was commonly put at 500. That was known to be an estimate and the U.S. Department of Defense finally completed a search of its records and reported that the actual number Americans killed was 109. These uniquely Iranian weapons apparently first appeared in 2006, but in small numbers. That increased so that by 2007 a lot more Explosively Formed Penetrators (EFPs) bombs were showing up. In January 2007 about thirty of them were used to attack U.S. troops. That rose to 65 by April, and 99 in July. By the time U.S. troops left in 2011 it was estimated that about 120 American troops had been killed by EFP and over 600 wounded. Hundreds of other American troops were believed to have died, usually at the hands of Iran supported Iraqi militias, but the use of EFPs was unambiguous and Iran was the only source.
The EFP, more commonly known as "shaped charges," have been around since World War II, when they were famously used in the bazooka and Panzerfaust (the model for the later RPG) portable anti-tank weapons. The EFP is an improved design over the basic shaped charge developed during World War II. Although most of the improvised explosive devices (IEDs) used in attacks against government and Coalition forces in Iraq have used conventional explosives (demolition charges, artillery shells, mines, etc.), a very small number have depended on EFPs.
Although EFPs are used in RPGs and similar missile weapons, they are not well suited for use in IEDs. This is fortunate, as they are much more dangerous. Reportedly EFPs cause about three times as many casualties per explosion as ordinary IEDs. A third of American combat deaths in July 2007 were caused by EFPs, and most of them are used by pro-Iranian Shia militias in Baghdad.
The problem with using EFPs as IEDs is that they have to be aimed. Most IEDs are designed to be detonated by trip-wire or similar mechanism or by remote command. Even a remotely detonated IED can cause serious damage, since it's the explosion is going to affect an area of some size. In contrast, an EFP has to hit something pretty much directly in order to have an effect. There is also a range problem, as EFP explosives have a very short effective range (a meter/three feet at most.) Moreover, since EFPs are intended for use against armor they have limited effectiveness against softer targets.
Iran was known to be smuggling many more EFPs into Iraq after 2007 and pro-Iranian terrorists used them in Shia neighborhoods of Baghdad. The EFP is most easily used in urban areas where U.S. vehicles can be easily stopped and where there is much less possibility of nearby civilians (usually Shia Arabs) being injured. After 2006 more of these EFPs were captured as they were being smuggled into Iraq. Examination of those, with the ones used in attacks, confirms that they come from the same source: a factory in Iran.