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– A roadside bomb that killed an American soldier in Iraq earlier this month was of a particularly lethal design not seen in six years, and its reappearance on the battlefield suggests that U.S. troops could again be facing a threat that bedeviled them at the height of the insurgency here, U.S. military officials said.

The device was of a variety known as an explosively formed penetrator, or EFP, a weapon notorious for its destructive and deadly impact on armored vehicles and the service members inside them, two U.S. military officials said.

EFPs were among the most lethal weapons faced by U.S. forces before a troop withdrawal in 2011. The devices were considered a hallmark of the Iranian-backed Shiite militias battling the U.S. occupation after the toppling of Saddam Hussein. But the technology used to make them proliferated, and cruder versions were also deployed by Sunni militants.

U.S. military officials were quick to stress that they had not determined who was responsible for the attack. The Islamic State of Iraq and Syria — the only threat to U.S. and Iraqi troops over the past three years — was not known to have previously used the weapons, the officials said, though they may have acquired the expertise to make them.

Dubbed “superbombs” because of their extraordinary lethality, EFPs are precision-made bombs with a copper or steel plate that is propelled in the form of a projectile whose high temperature and velocity can penetrate even the most heavily armored vehicles.

ISIS did not make any public claim of responsibility after the Oct. 1 attack, which killed Specialist Alexander W. Missildine and wounded another soldier, according to the U.S. military. At the time it was struck, Missildine’s vehicle was traveling south on a major road in Salahuddin Province, north of Baghdad.

Col. Ryan Dillon, a U.S. military spokesman in Iraq, said “investigations are continuing into the type and quality of the bomb to better determine where it originated. To say whether or not ISIS did it — we have not determined that yet. We are not ruling anything out,” he said.

The question of the type of bomb used and its origin is sensitive because it comes amid an intensifying drive within the Trump administration to counter the expansion of Iranian influence in the region in recent years.

It also coincides with threats from some of the Iranian-backed Shiite militias who have fought in uneasy alliance with the United States against ISIS but are making it clear that they want U.S. troops to leave now that the militant group is almost defeated.

The Asaib Ahl al-Haq group, which claimed responsibility for carrying out many of the attacks against U.S. troops in the years before 2011, said in a statement after the recapture of Mosul in July that the “resistance factions expect their return to the country after the defeat of ISIS.”

“If they are going to stay in any guise, the resistance factions will deal with them as occupiers just like they dealt with them before,” the statement said. Another group, Kataeb Hezbollah, issued a similar warning last month.