WASHINGTON — Iran may have backed off its support of Shiite insurgents in Iraq, the head of the Defense Department’s efforts to counter roadside bombs said Thursday.

U.S. officials have accused Iran of supplying Shiite militias with components for explosively formed penetrators, a particularly deadly type of roadside bomb that is much more powerful than bombs made from artillery shells.

While EFPs have accounted for about 5 percent of all roadside bombs, at times they have inflicted 35 percent of casualties from such devices, Army Lt. Gen. Thomas Metz, director of the Joint IED Defeat Organization, said Thursday.

In the past three months, the number of EFPs used in Iraq has dropped from between 60 and 80 per month to between 12 and 20 per month, he said.

When asked if that meant Iran has scaled back its support of anti-American groups in Iraq, Metz said the data suggest that.

"I’m not in the intel business, but that’s the conclusion I would draw," he said.

He did not provide any motivation for scaling back that support. "I don’t think I’ll venture into trying to figure out what’s in an Iranian’s head," he said.

Overall, the number of roadside bombs in Iraq has dropped from about 2,800 per month in 2007 to about 400 per month now, according to the task force.

Of those roadside bombs, about half are found before they go off. In the past few months, the number of U.S. fatalities from roadside bombs in Iraq has been in the single digits, he said.

Meanwhile, the Taliban and other insurgents in Afghanistan are using more roadside bombs. Metz said those attacks became the leading cause of casualties in Afghanistan about six months ago.

"They’ve come to the point that IED is probably the way they are going to influence us strategically," Metz said.