The Saudi-led coalition waging war on Yemen just carried out a horrific attack that killed dozens of people, including at least 29 children.

It’s entirely possible that the United States played a role in the Thursday bombing, but the US military doesn’t have any idea if that’s the case.

“We may never know if the munition [used] was one that the US sold to them,” Army Maj. Josh Jacques, a spokesperson for US Central Command, told me. “We don’t have a lot of people on the ground.” The military could conduct an investigation to find out if that’s the case, but it’s unclear if that probe would ever happen or how long it would take.

It’s also unclear if the US was involved in refueling planes for the attack, Jacques said, because the military doesn’t track where the coalition planes go. Another Pentagon spokesperson said that “US Central Command was not involved in the airstrike in Sa’ada.”

Here’s what happened: The Saudi-led coalition, which includes countries like the United Arab Emirates, bombed a school bus traveling through a busy market area in northern Yemen on Thursday. The bus was reportedly full of students on a recreational trip.

Estimates of the damage vary, but the International Committee of the Red Cross tweeted that its hospital in Sa’ada has received the bodies of 29 children under the age of 15. They’re treating 50 other people who were injured in the attack, including 30 children.

Children among the dead in attack on Yemen market: https://t.co/m75nOt3oBr pic.twitter.com/2ETgbXn49a — World News Tonight (@ABCWorldNews) August 9, 2018

The US backs the Saudi-led coalition’s war in Yemen by providing intelligence support and refueling airplanes, among other assistance. But it’s unclear if the US played a part in Thursday’s bombing by pumping gas into a coalition warplane, or if the coalition used US-made weapons to carry out the attack.

There’s one main reason for that, says Jacques: The US will refuel a coalition warplane when asked, but the US has no idea where the planes come from, where they’re going, or what their missions are. So the US military could have helped the reported coalition plane that bombed the bus, but they just don’t know in this particular case.

An exchange in March between Army Gen. Joseph Votel, the top US military official for the Middle East, and Sen. Elizabeth Warren (D-MA) during a Senate Armed Services Committee hearing supports that fact.

Here’s the relevant part, as reported by the Intercept:

“General Votel, does CENTCOM track the purpose of the missions it is refueling? In other words, where a U.S.-refueled aircraft is going, what targets it strikes, and the result of the mission?” Warren asked. “Senator, we do not,” Votel replied.

The US military is also not in the room when the Saudi-led coalition decides to conduct a strike, per Maj. Jacques. “At the end of the day, the Saudi-led coalition is responsible for their strikes,” he continued. A top spokesperson for the Saudi-led coalition defended the strike on Thursday by calling it a “legitimate military action.”

None of this sits well with Rep. Ro Khanna (D-CA), who has pushed legislation in Congress to end the Yemen war. He told me he finds the bus bombing “devastating,” that he plans to push for ending the war again, and hopes that there will be “more willingness to act.”

“This is the most devastating humanitarian crisis in the world and a moral stain on all of us in power,” Khanna said.

America is helping fuel the war on Yemen

While the bombing is shocking, it’s sadly not new. Experts tracking the Yemen war have seen horrific events like this since fighting began in 2015.

“No one can feign surprise when lots of civilians are killed anymore,” Scott Paul, a Yemen expert at the humanitarian group Oxfam America, told me. “It’s not just the heartbreaking slaughter of children; it’s the regular destruction of businesses, schools, hospitals, roads, and other infrastructure that keeps Yemenis alive.”

The war has claimed more than 13,500 lives, with more than 900,000 suffering from cholera. Roughly 20 million Yemenis need humanitarian assistance to meet basic needs — including food and water — out of a prewar population of 28 million.

These are all just estimates, though, as conditions on the ground are so bad that no one can do an official count. It’s also what makes it hard for the Pentagon to conduct an investigation into whether or not the coalition used its weapons in strikes, the Pentagon noted.

The Saudi-led coalition is fighting a rebel group called the Houthis, who staged a coup in 2014 to take over Yemen’s government. The war is the brainchild of Mohammed bin Salman, the 32-year-old crown prince of Saudi Arabia and defense minister. It’s part of his aggressive anti-Iran policy in the Middle East, which led him to intervene in Yemen in 2015 in support of the internationally recognized government against the Iran-backed Houthis.

Iran’s government is a Shia Muslim theocracy; Saudi Arabia’s government is a monarchy closely aligned with the country’s Sunni Muslim religious establishment. The two countries represent two ideological and political poles and have spent decades fighting each other for dominance in the Middle East and for the right to represent the Muslim world.

President Trump continues to support MBS politically, and the Pentagon backs his war in Yemen militarily. Without American pushback, the Saudi leader has little incentive to stop the relentless killing.

The US can’t move to end the war, says Paul, if “it keeps supporting one side of the conflict and shielding it from accountability.”