US officially blames Iran-backed group for drone strike, clearing way for retaliation

Is the U.S. on the brink of war with Iran? Officials hope not. But how President Joe Biden responds to this weekend's deadly attack on an American military base in Jordan could have far reaching implications in the region for years.

Experts say Biden's goal is to rein in Iran-backed militia groups operating out of Iraq, Syria and Yemen without plunging the Middle East into war.

Here's what to know:

The US officially pins blame on Islamic Resistance in Iraq

Soon after the drone attack in Jordan that killed three U.S. service members and wounded 40 more early on Sunday, Biden vowed that he'd hold those responsible and pinned the blame on Iranian-backed militants.

On Wednesday, the White House said U.S. intelligence was certain which militants were responsible.

Identifying the group responsible sets the stage for the attack, which officials say could happen at any time.

"We believe that the attack in Jordan was -- was a plan resourced and facilitated by an umbrella group called the Islamic Resistance in Iraq, which contains multiple groups, including Kataib Hezbollah," White National Security Council spokesperson John Kirby told reporters on Wednesday.

He also said the response would involve multiple targets and that "the first thing you see won't be the last thing."

A U.S. official said Iranian assets outside of Iran could be targets, with most strikes inside Syria.

Another official told ABC News the attack would be carried out "over the course of several days" on facilities that enabled the drone strike.

On the determination that the drone attack in Jordan was facilitated by the Islamic Resistance in Iraq, a senior State Department official said the U.S. is continuing to develop its assessment and may still conclude that a specific group or specific groups within that umbrella played more a direct role.

While Iran has denied involvement, the senior State Department official said the U.S. has made clear to Iran since Hamas' Oct. 7 terror attack, which sparked a war with Israel, that America will hold it responsible for the actions of its proxies.

Worries the US is acting too slowly

As the days ticked by since Sunday's attack, military experts surmised that the U.S. was using its time to gather assets for a more significant and complex response than seen previously. But Republican critics were quick to say Biden was losing ground, giving the Iranians time to evacuate potential targets or move their own military assets.

Sen. Roger Wicker of Mississippi, the top Republican on the Senate Armed Services Committee, said the U.S. attack should have already happened by now and that Iran's leadership and weapons caches should be hit directly.

"Every day that passes without a strong and unambiguous reprisal for the deaths of American service members invites our enemies – and allies – to question this administration's resolve," Wicker told ABC News in a statement.

'A pretty big target list'

Retired Gen. Robert Abrams, a former commander and an ABC News contributor, said the delay suggests to him that the U.S. response will be forceful and consequential and more widespread than recent strikes in the region.

"The longer it takes indicates to me that this is not going to be a little pinprick. A pinprick they could have done within six to 10 hours depending on available strike capability," he said.

Instead, Abrams said there's a "pretty big target list," including command-and-control nodes, storage facilities, any transit route for weapons or even an Iranian intelligence ship on the Red Sea.

"If they've got a smoking gun on who actually flew this suicide drone into Tower 22, you can expect that that [command-and-control] network, the emitters where the nodes, where commands were coming from …those are all going to be fair game," he said.

For its part, Iran has warned that its own response will be "decisive and immediate."

"The U.S. should stop using the language of threats and pinning the blame on others and rather focus on a political solution. Iran's response to threats will be decisive and immediate." Iran's Foreign Minister Hossein Amir Abdollahian said, according to the Islamic Republic News Agency.

Triggering a retaliatory response from Iran and spurring a broader conflict is of obvious concern for the U.S, but experts and government officials say America must act.

"We've been trying to determine Iran's red line for many years," Abrams said. "And we haven't found it."

Editor's note: This story has been updated to correct Retired Gen. Robert Abrams' name.