A complex air war over Libya has killed hundreds of innocent civilians, and possibly many more, since the U.S.-led NATO intervention in the North African state began in 2011. And almost no one outside of the war-torn country has even noticed. The U.S. government, for its part, increasingly seems to prefer it that way.

Manned warplanes and drones from four foreign countries and various Libyan factions conducted at least 2,158 air strikes between September 2012 and mid-June 2018, according to a new report from the Washington D.C. based New America Foundation and Airwars.org, an air-strike-tracking project affiliated with the University of London.

The strikes killed as many as 387 bystanders and wounded up to 524, according to the report. That amounts to one civilian death every 5.5 air raids, on average, and as the report points out, "No nation or local group has stated responsibility for any of these civilian deaths.”

Compared to, say, Yemen, the rate of civilian casualties might seem low. In Yemen, the U.K.-based Bureau of Investigative Journalism counted 217 air strikes since the Saudi-led intervention in that country beginning in early 2015, resulting in as many as 52 civilian deaths. That's an average of one death every 4.2 strikes.

The seemingly lighter bloodshed in Libya raised researchers' suspicions. "Reported civilian harm from air strikes in Libya is relatively low when compared to higher-intensity conflicts in, for example, Iraq, Syria or Yemen," the Airwars.org and New America Foundation report notes.

The current civil war in Libya began in early 2011, when rebels attacked the regime of long-time dictator Muammar Gaddafi. NATO intervened in March of that year, officially aiming to protect civilians caught in the crossfire. Rebels captured and killed Gaddafi in late October. The NATO mission ended a few days later.

Shortly thereafter, Libya fractured. Two main rival forces — the Government of National Accord in the west and the Libyan National Army in the east — began battling for control of key coastal towns and strategic oil facilities in Libya's interior. Taking advantage of the chaos, the so-called Islamic State and Al Qaeda both have attempted to gain footholds in the country.

The overlapping problems of civil war and terrorism have drawn a confusing mix of foreign forces into Libya. France, Egypt and the United Arab Emirates all have backed the LNA with air strikes. Meanwhile the United States has repeatedly struck suspected terrorists in the country.

Drones play a major role. After being denied U.S.-made Predators, the UAE purchased weapons-capable Wing Loong drones from China and, in 2016, deployed them alongside manned attack aircraft at a base in eastern Libya. From there, the Emirati aircraft fly top cover for LNA troops.

The LNA itself operates Iranian-made Mohajer surveillance drones, apparently provided by either Iran or Sudan.

But it's the United States that has made killer drones a regular sight over Libya. In early 2016, the Government of National Accord in western Libya launched an operation to dislodge ISIS fighters from the coastal city of Sirte. U.S. forces came to the GNA's aid.

The battle raged for months. Most Americans never understood the full extent of the U.S. involvement in the fighting. The U.S. Navy and Marine Corps released photos depicting manned fighters and attack helicopters operating off the Libyan coast, but those missions represented a small fraction of the estimated 495 American airstrikes in Sirte.

Two-thirds of the Sirte strikes were conducted by U.S. Air Force Reaper drones, Chris Woods, the author of Sudden Justice: America's Secret Drone Wars, told The Daily Beast. The intensity of the Reaper ops was the "biggest surprise" of the Libyan air war, Woods said.

The American air raids in Sirte killed as many as 20 civilians, according to the Airwars.org and New America Foundation report. But U.S. Africa Command denied harming any civilians in any of its hundreds of airstrikes in Libya. "Africa Command has many processes in place to ensure the safety and protection of the local population remains a top priority," the command told Airwars.org and New America.

Marine general Thomas Waldhauser, the head of Africa Command during the Sirte battle, told reporters that he personally witnessed drone operators halt attacks that might have harmed bystanders in the Libyan city. "I witnessed the operators either pull a missile off a target or just stop their mission completely when someone or something came into the view of the camera that we weren't sure who or what it was," Waldhauser said.

But the Airwars.org and New America Foundation report stresses the difficulty of verifying the Pentagon's, and its own, claims regarding civilian casualties. "Libya lacks local monitors such as the Syrian Network for Human Rights, which assesses civilian harm in Syria."

After Sirte, Africa Command grew more reluctant to discuss air strikes in Libya, an apparent shift in policy that has made it possible for the Pentagon "to conceal the extent of its operations in Libya," according to the report.

Airwars.org and New America found evidence of eight U.S. air strikes in Libya after the Sirte battle ended in January 2017, but Africa Command initially disclosed only four of the attacks in official press releases. It was only after researchers from Airwars.org and New America directly contacted Africa Command that the military issued statements describing the four "missing" air strikes, the most recent of which took place on June 13.

“We do acknowledge all strikes — either by press release or as a response to query,” Samantha Reho, an Africa Command spokesperson, told The Daily Beast. “When we use the response to query format, it is because of operational security, force protection or diplomatic sensitivities. Our goal is always to be as transparent as possible taking into account any necessary operational precautions.”

The U.S. raids in Sirte accounted for more than half of the record 1,000 air strikes in Libya in 2016. Researchers counted just under 700 air raids in 2017 and around 100 by mid-June 2018.

The four countries and two factions responsible for the overwhelming majority of the air raids together have claimed responsibility for just half of the attacks, according to the report. The media unwittingly enable this failure of accountability. "A lack of international reporting on the air war has helped to obscure the fact that the countries involved in Libya elect not to report their air strikes," the report explains.

Likewise, no country or faction has assumed any responsibility for any of at least hundreds of civilian deaths that Airwars.org and New America attribute to airstrikes in Libya.