CNN — The US believes an umbrella group of militants called Islamic Resistance in Iraq was behind the drone attack in Jordan that killed three American service members, the White House said Wednesday, its first formal attribution for the incident.

“We believe that the attack in Jordan was planned, resourced and facilitated by an umbrella group called the Islamic Resistance in Iraq, which contains multiple groups including Kata’ib Hezbollah,” National Security Council spokesman John Kirby said.

He stopped short of assigning exact blame on Kata’ib Hezbollah, saying it was not the only group responsible for previous attacks on US bases.

“This certainly has the earmarks of the kinds of things that Kata’ib Hezbollah does,” he said, adding, “The attribution that that our intelligence community is comfortable with is that this was done by the umbrella group called the Islamic Resistance in Iraq.”

The attack was the most serious of scores of attacks that have targeted US forces in the Middle East since the October 7 attack by Hamas on Israel, and the first time US troops have died under direct fire in the Middle East since the attack.

The killed servicemembers were identified as Sgt. William Rivers, 46, of Carrollton, Georgia; Sgt. Kennedy Sanders, 24, of Waycross, Georgia; and Sgt. Breonna Moffett, 23, of Savannah, Georgia. Sanders and Moffett were posthumously promoted from the rank of specialist to sergeant, the Army Reserve said.

Biden spoke with the families of each of the three servicemembers on Tuesday and will attend a dignified transfer at Dover Air Force Base on Friday, officials said.

In addition to the three killed, more than 40 US troops were injured. Three of the injured troops were transferred to a hospital in Germany on Tuesday evening for further treatment. All three are reported to be stable, but one is in critical condition

Kirby said as the US was preparing to respond to the attack, there would be multiple phases of a counterattack.

“We will respond on our own time on our own schedule,” he said, adding later: “The first thing you see won’t be the last thing.”

He said US intelligence was monitoring for signs of groups in the region moving resources around ahead of the US counterstrike.

Biden told reporters on Tuesday that he had decided how to respond to the strike, but remained vague on what exactly the response would be.