The U.S. Air Force’s A-10 Thunderbolt attack aircraft is responsible for the highest number of friendly-fire and civilian deaths of any aircraft flown by the U.S. military since 2001, according to recently declassified government documents. The revelation comes after the aircraft was saved from being retired last year by a group of powerful supporters in Congress, led by Republican Sen. John McCain. 

The 40-year-old aircraft hasn't been manufactured in decades. Built during the Cold War to stop Soviet tanks, it's an ungainly, slow jet that lacks the technical sophistication of today's warplanes, or the lobbying heft of aerospace companies such as Lockheed Martin, maker of the ultra-expensive F-35. But troops loved it for its vast firepower and ability to annihilate enemy fighters in close quarters, and it earned a reputation in the past decade of American wars as a great friend to soldiers on the ground -- until today. 

The revelation also may increase pressure on the Pentagon to scrap it as the 2016 defense budget makes its journey through Congress.

The documents, which were obtained by USA Today, show that the A-10 was responsible for 10 friendly-fire deaths in Afghanistan since 2001, double the amount of the B1-B bomber, and as many as 35 civilian deaths in Afghanistan compared with 19 deaths for the similar AV-8B Harrier jet. There have only been 45 friendly-fire deaths in total between the Air Force, Navy and Marines out of 140,000 missions flown in the wars in Iraq and Afghanistan, according to figures cited by USA Today. A-10s killed about one-fourth of all soldiers who died by friendly fire, although they flew much less than one-fourth of the air missions in those wars. 

The news will leave many of the aircraft’s supporters puzzled as to why an aircraft that has been lauded as a lifesaver for troops on the ground for 40 years is in fact the most dangerous.

One reason, said Richard Aboulafia, an aviation expert and vice president of analysis at Teal Group, a Virginia-based consultancy, is the nature of how the A-10 engages in battle.

“Of all the close air support platforms, the A-10 flies and attacks targets closest to friendly troops,” said Aboulafia. “It also uses a cannon that's not precisely targeted. So, it's being compared with high-altitude close air support aircraft that only attack with precision guided munitions.”

One of the key arguments in the debate over the A-10 has been that it offers more precise on-the-ground targeting than high-altitude jets that are said to fly too fast and too high to hit small targets. Troops rely on calling in support from an A-10 that can arrive fast, loiter over the battlefield at low altitude and eliminate threats as they appear. Large bombers can only hit when they have precise coordinates, which is hard to do in the heat of combat. 

Yet, faced with dwindling budgets, many top officers in the U.S. Air Force want to phase out the A-10 completely by 2019 to invest resources in more modern jets. The A-10, however, proved its usefulness again recently in the fight against the Islamic State group, flying 14 percent of all missions in Iraq.

High-altitude aircraft are able to hit targeted buildings and large moving convoys of the Islamic Sate, but the A-10 is able to fly just above the ground, able to seek out smaller and harder-to-hit targets with its 30mm cannon that can fire as many as 3,900 depleted uranium shells per minute. However, at a top speed of 440 mph (700 kph) it is an easier target than most aircraft, but it has a thick, armored airframe that protects it from enemy fire.

In recent years the aircraft has come to symbolize the fraught relationship between the Pentagon and Congress, since sequestration cuts were introduced in 2011. The Air Force says it would save upwards of $4.2 billion if the A-10 was scrapped.

"The A-10 has been in service for 40-plus years," said Lt. Col. Chris Karns, an Air Force spokesman, to USA Today. "While the A-10 and its airmen have a long and proud history, fiscal realities and the significant cost savings associated with A-10 divestment are resulting in tough decisions."

But the A-10, affectionately known as the Warthog, still has powerful supporters in Congress. McCain and Sen. Kelly Ayotte, a New Hampshire Republican whose husband flew the A-10, have managed to keep the plane's critics at bay by convincing Congress that it was an essential piece of equipment. Both were unavailable for comment on this story.

Dustin Walker, a spokesman for the Senate Armed Services Committee, said the report's findings mean little without understanding the background of the aircraft’s mission and the circumstances in which the friendly-fire incidents occurred. "While any loss of life is a great tragedy, in the context of tens of thousands of Air Force combat missions, this data is inconclusive and statistically insignificant to determining which weapon system is most effective in its primary mission, or at avoiding civilian casualties or friendly-fire incidents," Walker told USA Today.

The friendly-fire report may give new life to A-10 opponents who say the Air Force needs to terminate it in order to divert maintenance personnel to the new F-35. Dakota Wood, a senior research fellow of defense programs at the conservative think tank Heritage Foundation, said the needs of the F-35 will most likely determine the future of the A-10, not friendly-fire statistics. “I just don’t think [the friendly-fire report] is going to be influential in the debate at all,” said Wood. “The decision to retire the A-10 is purely a budget-driven exercise to free up money for the F-35A.

But as chairman of the Armed Services Committee, it's McCain who may have the final say over its future, Wood said. “McCain’s argument will be, how can the F-35A, costing what it does, adequately provide the same kind of support that the A-10 has given over the last four decades?” 

The 40-year-old Warthog may after all live to fight another year.