According to a Syrian monitor group, warplanes believed to be part of the U.S.-led anti-ISIS coalition bombed and killed 18 people in the Idlib province town of Atmeh on Tuesday night, close to the Turkish border.

Eight of the alleged fatalities were civilians, including women and children—among them, five young sisters and a three-person family of internally displaced persons. The remaining 10 were said to be fighters belonging to the independent Free Syrian Army brigade known as Jaysh al-Sunnah. One of the brigade’s main bases was apparently destroyed.

If confirmed, this attack would not only scandalize an already fraught year-long campaign against the transnational terror group; it would undoubtedly squander whatever’s left of the trust held by anti-Assad rebels toward Washington. Particularly after the Pentagon this week quietly announced that its $500 million program to “train and equip” rebels to fight the so-called Islamic State widely known as ISIS was being largely sidelined, following the fiasco of seeing all of the 54 initial graduates run off or get killed or captured by al-Qaeda.

The Pentagon confirmed to The Daily Beast that a target “near Aleppo” was struck on August 11, although described it as an ISIS “staging area.” (Ragip Soylu, a Turkish journalist, had earlier obtained confirmation from the coalition that Atmeh is classified as “near Aleppo.”) U.S. Central Command said that it’s currently assessing whether to open an investigation into the bombing, contingent on what eyewitness statements, photographs, and other documents reveal about the sortie.

Meanwhile, U.S. Air Force Major Genieve David, a spokesperson for CENTCOM, issued the following statement: “We are aware of the reports surrounding Atmeh and are looking into them. We take all reports of non-combatant casualties seriously and look into each one received or reported. We apply very rigorous standards in our targeting process to avoid or minimize civilian casualties in the first place—to include analysis of available intelligence to selection of the appropriate weapons to meet mission requirements with the goal to minimize the risk of collateral damage and, in particular, potential harm to non-combatants.”

Arabic social media was awash with rumors of the Atmeh strike, encouraged by a series of heartrending interviews conducted by Syrian media activist Hadi al-Abdallah. In one, he talks to a man who says he is the grandfather of the five dead sisters, who ranged in ages from 4 to 10. “Are these girls terrorists?,” the grandfather asks. “Are they carrying machine guns? May God take revenge!”

He recounts to al-Abdullah that three years earlier, he’d given Jaysh al-Sunnah, a brigade originally from Homs province, a plot of land typically free of Assad regime bombardment to build a mortar shell workshop. (In a separate video, the “technical supervisor” of the facility claims that six missiles in total demolished it.)

Muna Hallaj, who says she is the mother of the girls, tells al-Abdullah: “My kids were playing on the front porch, and I was going to milk the cow when suddenly bombs started falling. The house just fell apart on our heads! I'm sorry, I cannot speak.”

Then a man identified as the father of the dead girls is queried. “What do we have to do with terrorism?” he asks. “Two women were killed like our neighbors, five daughters were killed, and a man was killed. Two women and two children were wounded. We were suddenly bombed three times! May God take revenge on everyone in the coalition. They are worse than Bashar and they do not want Bashar to go.”

Abdullah also interviews Abo Ahmed, a spokesman for Jaysh al-Sunnah, the FSA brigade allegedly hit by the bombing. “As if the Syrian people’s catastrophes were not enough!” Abo Ahmed says. “Jaish al-Sunna is a purely Homsi group that arose spontaneously. We are not ideologues. Most of us came north to escape the regime bombing of Homs. We thought the regime would not bomb here because it was too close to Turkey. We never expected the coalition to strike.”

Abo Ahmed also suggests that the reason for the raid was to halt the progress of Jaysh al-Fatah, or the Army of Conquest, an umbrella faction of which Jaysh al-Sunnah is a part. The Army, backed by Saudi Arabia, Qatar, and Turkey, has lately been battling both the Assad regime and ISIS, although it hardly curries favor in the West given that one of its more powerful constituent groups is Jabhat al-Nusra, the official al-Qaeda franchise. Even so, there is no precedent or pretext for the U.S. to target the FSA, virtually all units of which have fought alongside al-Nusra.

Robert Ford, the former U.S. ambassador to Syria, said that news of the airstrike was taken up by al-Nusra, which “bitterly criticized the Americans and the Syrians who work with the Americans.”

“The Americans need to get a strategy, and then they need to get the tactics right,” Ford told The Daily Beast. “The strategy they need to think about is: What are they trying to achieve? How do they undermine the [extremist] recruitment problem?… Big silence, not even expressing regret at civilian casualties, isn’t going to help reduce recruitment into extremist groups.”

Ford also expressed consternation at why an airstrike was even conducted in the area, where a group called Jaysh al-Sunna operates. “It’s not Nusra, it’s not a Nusra affiliate. There is not an ISIS staging area near [the region the coalition struck]. They are well to the east.”

“If true, the repercussions depend partly on who did and whether it was deliberate,” said Faysal Itani, a resident fellow of the Rafik Hariri Center for the Middle East at the Atlantic Council think tank. “The U.S. military rarely makes such grievous targeting errors, so if this is a deliberate air strike on a mainstream rebel group it has not designated as terrorists, this suddenly broadens the definition of U.S. enemies in Syria. No one will know whether they are legitimate targets. Conspiracy theories will abound, and trust between factions will break down, and perhaps U.S.-Turkish cooperation will, too.”

As of this writing, it is unclear not only what aircraft allegedly bombed Atmeh but where they flew from. Eyewitnesses say that warplanes approached the town from Turkish airspace. And while it’s true that Wednesday saw the first strikes on Syria to be launched from Turkey’s Incirlik airbase, which Ankara recently authorized the coalition to use, a Turkish Foreign Ministry source said that no manned or unmanned aircraft had left for Syria on the previous day. If Atmeh was bombed by Syrian jets, then they will have risked being shot down by Turkey, which has long maintained a de facto policy of deterrence so close to its southern border.

It may take quite a while for the full facts of the Atmeh attack to be disclosed. Of the roughly 4,000 strikes conducted during Operation: Inherent Resolve, the coalition has only conceded killing two civilians, last May—both children. Privately, however, U.S. officials concede the figure is undeniably higher. But with no ground troops to assess strike damage, CENTCOM has no way to confirm the effects of its strikes. There are currently three cases under investigation.

One report, put out by the not-for-profit organization Airwars.org, which tracks and archives the coalition war against ISIS, has found 459 casualties because of coalition airstrikes.

Some Syrian-American activists are not waiting for official corroboration as to what may have just happened. “It seems that the Obama administration is unable to distinguish friend from foe,” Mohammed Alaa Ghanem, the senior political adviser and government-relations director of the Syrian American Council, an activist group, told The Daily Beast. “While the State Department accuses Assad of being ISIS’s airforce, the administration refuses to target Assad and coalition warplanes bomb children and moderate rebels far from ISIS territory. The White House should apologize and offer reparations to the victims’ families. At this rate, the ISIS fight will be a ‘strategic stalemate,’ as the CIA put it, for a long time.”

—With additional reporting from Nancy Youssef and Tim Mak