U.S. forces have captured a man who may help unravel an elite, highly skilled insurgent group whose rocket-propelled bombs have emerged as the biggest threat to U.S. troops, the top commander of U.S. forces in Baghdad said Friday in an Associated Press interview.

Army Maj. Gen. Jeffery Hammond, speaking in his office at this U.S. military compound west of the capital, said the shadowy group is small and has demonstrated an unusual degree of military skill and cunning.

"It's the greatest threat right now that we face," Hammond said, adding that the threat is less significant for the Iraqi population because the bombs are targeting only U.S. and Iraqi security forces.

At least two U.S. troops have been killed this year by the weapon. Hammond said he did not know whether it is linked to Iran, although some have speculated on a connection because the 107mm rockets used were made in Iran and because the group seems to be part of a Shiite militia.

The U.S. military has dubbed the weapon an "improvised rocket-assisted mortar." The Iraqis call it a "lob bomb" because it essentially is a rocket-fired mortar that is lobbed — several in one firing — a short distance from the back of a truck. It is detonated in some cases by a signal from a cell phone.

It's sort of an airborne I.E.D., the roadside bombs that during the course of the war have been the biggest killer of U.S. troops.

Hammond, who spent part of his day Friday discussing the problem with troops at a remote outpost in the rural outskirts of southwest Baghdad, said in the AP interview later that it has been particularly difficult to figure out who is behind the bomb group.

On Thursday U.S. forces snatched a person who may help unlock the mystery, Hammond said.

"We think we might have picked up a guy that could lead us — could be a big lead in this," he said.

Hammond said he has told his subordinate commanders that the group is "sort of the Delta Force" of the broader insurgency, alluding to the highly secretive force in the U.S. military that is trained to conduct counterterrorism missions and in some cases fight without revealing a U.S. government link.