In Iraq and Afghanistan, homemade, low-tech bombs, often hidden along roadsides, have been some of the deadliest threats to United States soldiers. In Afghanistan alone, these improvised explosive devices, or IEDs, were responsible for 1,290 of the 2,477 U.S. and coalition casualties since 2001, according to iCasualties.org, which tracks troop deaths.

Now, IEDs could be coming to U.S. shores.

According to Lt. Gen. Michael Barbero, head of the military’s Joint Improvised Explosive Device Defeat Organization, some of the same terrorists who amassed the know-how on building IEDs are setting their sights on the U.S.

“Today’s IEDs are relatively simple, low-tech devices, which routinely use command wire, pressure plates, or radio-controlled triggers,” Barbero wrote in written testimony released Thursday ahead of a closed hearing of a subcommittee of the House Homeland Security Committee. “Many readily available components such as cellphones, agricultural fertilizers and simple electronic transmitters and receivers have legitimate commercial uses, but are easily and increasingly adapted for illicit purposes in manufacturing IEDs.”

Barbero cited the case of an Iraqi named Waad Ramadan Alwan who moved to Kentucky in 2009 after winning a visa as a political refugee. According to a December 16, 2011, plea agreement, Alwan was an Iraqi insurgent working between 2003 and 2006 to plant IEDs on roads traveled by U.S. troops. In 2010, in Bowling Green, Ken., Alwan began to train another man in how to make them, making diagrams of the bombs and giving oral instructions on how to assemble the devices, according to the document.

In his testimony, Barbero said the military provided key intelligence to the FBI in the Alwan case. He said their cooperation was an example of weapons technical intelligence, a process of identifying the markings of signatures of explosives and matching with other kinds of data, working to neutralize a threat.

“The domestic IED threat from both homegrown extremists and global threat networks is real and presents a significant security challenge for the United States and our international partners,” Barbero said in his written testimony. He said that since 2007, on average there are 500 attempted IED detonations per month outside of Iraq and Afghanistan. After those countries, IEDs are most prevalent in Colombia, a country that has fought against a drug-fueled insurgency since the 1990s.

After opening statements on Thursday, the committee went into closed session, citing the sensitivity of intelligence surrounding the IED threat to the U.S.

After killing Osama bin Laden, the CIA and U.S. military knocked out a succession of senior al Qaeda leaders with the intelligence haul from his compound. In his State of the Union speech this year, President Obama said, “For the first time in two decades, Osama bin Laden is not a threat to this country. Most of al Qaeda’s top lieutenants have been defeated. The Taliban’s momentum has been broken.”

Nonetheless, al Qaeda has adapted, according to congressional testimony from the Director of National Intelligence. Instead of relying on seasoned veterans to plan elaborate attacks, the group’s new tactics emphasize recruiting Americans and Westerners and planning lower-tech attacks, say experts. Today, al Qaeda shares best practices for terrorism through online chat rooms and an Internet publication known as Inspire.

One homegrown threat was Faisal Shahzad, a Pakistani-American, who pled guilty to assembling a homemade bomb that nearly exploded in Times Square on May 1, 2010. Shahzad, who was sentenced to life in prison, had reportedly attended a terrorist training camp in the province of Waziristan.

Shahzad and another would-be bomber, Najibullah Zazi, an Afghan-American who attempted to blow up the New York City subway in 2009, were able to obtain the ingredients for their bombs over the counter, according to court documents.

The military and Department of Homeland Security are particularly concerned about the availability of ammonium nitrate, a fertilizer that was also the primary ingredient of the Oklahoma City bombing in 1995. The title of the hearing Thursday was “Securing Ammonium Nitrate: Using Lessons Learned in Afghanistan to Protect the Homeland from IEDs.”

Rep. Dan Lungren, a California Republican and chairman of the subcommittee that held the hearing, said in an interview that he did not yet endorse tighter restrictions on selling the fertilizer. For now, he said, he wanted to work with the chemical industry and law enforcement to figure out ways track its sale so it wouldn’t be used for terrorism.

“The IED is the most deadly form of attack on our troops in the war zone,” Lungren said. “That has increased over time. Is there any question that those who are dedicated to killing Americans would not exploit vulnerabilities in the United States to use IEDs here?”