WASHINGTON — U.S. forces may have mistaken an enemy drone for an American one and let it pass unchallenged into a desert base in Jordan where it killed three U.S. troops and wounded dozens more, officials said Monday.

Details of the Sunday attack emerged as President Joe Biden faced a difficult balancing act, blaming Iran and looking to strike back in a forceful way without causing any further escalation of the Gaza conflict.

As the enemy drone was flying in at a low altitude, a U.S. drone was returning to the small installation known as Tower 22, according to a preliminary report cited by two officials, who were not authorized to comment and insisted on anonymity,

As a result, there was no effort to shoot down the enemy drone that hit the outpost. One of the trailers where troops sleep sustained the brunt of the strike, while surrounding trailers got limited damage from the blast and flying debris.

Aside from the soldiers killed, the Pentagon said more than 40 troops were wounded in the attack, most with cuts, bruises, brain injuries and similar wounds. Eight were medically evacuated, including three who were going to Landstuhl Regional Medical Center in Germany. The other five, who suffered minor brain injuries, were expected to return to duty.

The preliminary conclusion was first reported by The Wall Street Journal.

Asked if the failure to shoot down the enemy drone was “human error,” Pentagon spokeswoman Sabrina Singh responded that the U.S. Central Command was still assessing the matter.

The Pentagon identified those killed in the attack as Sgt. William Jerome Rivers, 46, of Carrollton, Ga.; Spc. Kennedy Ladon Sanders, 24, of Waycross, Ga., and Spc. Breonna Alexsondria Moffett, 23, of Savannah, Ga.

The three U.S. Army Reserve soldiers were assigned to the 718th Engineer Company, 926th Engineer Battalion, 926th Engineer Brigade in Fort Moore, Georgia.

The explanation for how the enemy drone evaded U.S. air defenses came as the White House said Monday it’s not looking for war with Iran even as Biden vows retaliatory action. The Democratic administration believes Tehran was behind the strike.

Biden met with national security advisers in the White House Situation Room to discuss the latest developments and potential retaliation.

“There’s no easy answer here,” said National Security Council spokesman John Kirby. “And that’s why the president is meeting with his national security team weighing the options before him.”

The brazen attack, which the Biden administration blames on Iranian-based proxies, adds another layer of complexity to an already tense Mideast situation as the Biden administration tries to keep the Israel-Hamas war from expanding into a broader regional conflict.

“The president and I will not tolerate attacks on U.S. forces, and we will take all necessary actions to defend the U.S. and our troops,” Defense Secretary Lloyd Austin said as he met at the Pentagon with NATO Secretary General Jens Stoltenberg.

The drone attack was one of dozens on U.S. troops in the Middle East since Hamas launched attacks on Israel on Oct. 7, igniting the war in Gaza. But it’s the first in which American service members have been killed.

Biden promised on Sunday to “hold all those responsible to account at a time and in a manner (of) our choosing” but said the U.S. wasn’t seeking to get into another conflict in the Middle East.

Kirby also made clear that American patience has worn thin after more than two months of attacks by Iranian proxies on U.S. troops in Iraq, Syria and Jordan and on U.S. Navy and commercial vessels in the Red Sea. The proxy groups — including Yemen’s Houthi rebels and Iraq based Kataeb Hezbollah — say the attacks are in response to Israel’s ongoing military operations in Gaza.

“We are not looking for a war with Iran,” Kirby told reporters. “That said, this was a very serious attack. It had lethal consequences. We will respond, and we respond appropriately.”

Iran on Monday denied it was behind the Jordan strike.

“These claims are made with specific political goals to reverse the realities of the region,” Iran’s state-run IRNA news agency quoted foreign ministry spokesman Nasser Kanaani as saying. Iran regularly denies involvement in attacks linked back to it through the militias it arms across the wider Mideast.

Kirby said that U.S. officials are still working through determining which militant group was behind the attack. He noted that Iran has longed equipped and trained the militias.

Republicans have laid blame on Biden for doing too little to deter Iranian militias, which have carried out approximately 165 attacks on U.S. troops in the region since the start of the war.

Republican presidential front-runner Donald Trump on Sunday called the attack “yet another horrific and tragic consequence of Joe Biden’s weakness and surrender.”

The attack hit a U.S. military desert outpost in the far reaches of northeastern Jordan known as Tower 22. The installation sits near the demilitarized zone on the border between Jordan and Syria along a sandy, bulldozed berm marking the DMZ’s southern edge. The Iraqi border is only 10 kilometers (6 miles) away.

The base began as a Jordanian outpost watching the border, then saw an increased U.S. presence after American forces entered Syria in late 2015. The small installation includes U.S. engineering, aviation, logistics and security troops, with about 350 U.S. Army and Air Force personnel deployed.

Iraq’s government condemned the drone strike. Spokesman Bassem al-Awadi said in a statement that Iraq was “monitoring with a great concern the alarming security developments in the region” and called for “an end to the cycle of violence.” The statement said that Iraq is ready to participate in diplomatic efforts to prevent further escalation.

An umbrella group for Iran-backed factions known as the Islamic Resistance in Iraq has claimed dozens of attacks against bases housing U.S. troops in Iraq and Syria since the Israel-Hamas war began. On Sunday, the group claimed three drone attacks against sites in Syria, including near the border with Jordan, and one inside of “occupied Palestine” but so far hasn’t claimed the attack in Jordan.

John Bolton, who served as national security adviser to Trump, said Iran hasn’t paid a price for the havoc that its proxies have unleashed in the region. He suggested the Biden administration could send a strong message to Tehran with strikes on Iranian vessels in the Red Sea, Iranian air defenses along the Iraqi border, and bases that have been used to train and supply militant groups for years

“So until Iran bears a cost, you’re not going to reestablish deterrence, you’re not going to put the belligerence on a downward slope.”

The attack came as U.S. officials were seeing signs of progress in negotiations to broker a deal between Israel and Hamas to release the more than 100 remaining hostages being held in Gaza in exchange for an extended pause in fighting. While contours of a deal under consideration would not end the war, Americans believed that it could lay the groundwork for a durable resolution to the conflict.

Top U.S., Israeli, Egypt and Qatari officials held talks on Sunday in France about an emerging framework for a hostage deal. Israel said “significant gaps” remain but called the talks constructive and said they would continue in the week ahead.