Joint Chiefs Chairman Gen. Mark Milley said the attack, which also killed a coalition member and wounded 14 others, was carried out by "Shia militia groups." He did not name the group responsible but said "we have pretty good confidence we know who did this."

Earlier on Thursday, Central Command chief Gen. Frank McKenzie told senators that Iran-backed Kataib Hezbollah is the only Shia militia group known to have conducted "an indirect fire attack on this scale against U.S. coalition forces in Iraq." The group attacked a military base in Kirkuk, Iraq, in December that killed a U.S. contractor and set off a series of responses that brought the U.S. and Iran to the brink of war.

Wednesday’s attack came from an improvised truck launcher that fired off 30 Katyusha rockets, 18 of which landed on Camp Taji, a base just north of Baghdad that houses U.S. and coalition forces. None of the incoming rockets was shot down by any air defense system, a U.S. official said.

Milley said the U.S. was able to capture the truck with Iraqi security forces and has "good indications" who fired the rockets based on forensic evidence."

"The groups that were responsible will be held accountable," he said.

When asked why the U.S. didn't intercept the incoming rockets, Milley responded that the base has no defenses for those types of weapons.

“On that base with these type of rockets, no they were not intercepted. It's not a function of failure. There's not a system there to defend against those types of rockets," he said.

The U.S. is moving air and ballistic missile defense systems into Iraq to protect against a potential Iranian attack following the escalation in January, McKenzie told the House Armed Services Committee on Tuesday.

Following the attack, reports emerged of airstrikes on Iran’s Islamic Revolutionary Guard Corps positions on Abu Kamal near the border between Iraq and Syria. However, the U.S. official said those strikes were not conducted by the United States. It is possible the Israeli military carried out the strikes.

One former Trump administration official said it is likely Wednesday’s attack was conducted with the “knowledge and support of the IRGC.”

“Although some stated they believed the killing of [Iranian Gen. Qassem] Solemani would prevent further attacks, many more believed it would not and that this is the start of their response,” the former official said.

The Pentagon is likely preparing options to present to the president for a response, ranging from direct attacks against the militia group that carried out the rocket strike, targeting IRGC operatives in Iraq or Syria, and perhaps even direct attacks on Iranian military in Iran. The military may also be considering “covert actions,” which could help reduce the chances of escalation.

