WASHINGTON >> All the Navy recovered after Lt. Nathan Poloski’s fighter jet collided in midair with another jet on a training mission in the western Pacific in September were his flight helmet and a few bits of debris. The 26-year-old pilot, who was deployed aboard the aircraft carrier Carl Vinson, died one month before the ship steamed into the Persian Gulf and began flying combat missions against the Islamic State.

Poloski’s body and his F/A-18C Hornet were never found in waters nearly 3 miles deep. The other Hornet pilot was rescued after ejecting from his burning jet. Navy officials concluded that the crash shortly after takeoff was a tragic accident and assigned no blame.

Since the Sept. 11, 2001, terrorist attacks, the United States has focused largely on the thousands of U.S. casualties suffered in the grinding ground combat in Iraq and Afghanistan. But while tiny in comparison, the number of fatal aviation accidents — in combat and in training — illustrates the everyday hazards facing U.S. military pilots and aircrews around the globe.

The Navy’s investigation into Poloski’s fatal crash — a copy of which was obtained by The New York Times through a Freedom of Information Act request after it was completed last month — reveals new details about the collision as well as larger insights into the dangerous business of carrier operations, even when a ship and its aerial armada are not at war.

What remains a mystery, though, is exactly what caused the accident. It was a clear afternoon with good visibility. Both pilots were healthy, properly rested and under no unusual stress. Investigators found no mechanical problems.

The Navy’s top aviator concluded in his review of the eight-page accident report that the two pilots, and several others in the vicinity, should have exercised more of what the military calls situational awareness, or SA — in this case, relying not only on an instrument-packed cockpit but also on looking outside to spot a looming catastrophe.

“While there is no definitive evidence to suggest either pilot’s SA or lack thereof directly contributed to this incident, greater SA by all parties may have prevented the collision,” Vice Adm. Mike Shoemaker, the senior aviator and an F/A-18C pilot, wrote on April 20 in closing the inquiry. The admiral broadened the inquiry’s original conclusion that admonished only Poloski for lacking situational awareness.

Poloski, a Naval Academy graduate from Lake Arrowhead, Calif., who was on his first deployment, became one of four naval aviators to die in accidents in 2014 — a roughly average figure for the Navy in the past several years. Two airmen have died so far in the 10-month campaign against the Islamic State: an Air Force pilot whose F-16 crashed in Jordan in November and a Marine who died in October after bailing out of an Osprey aircraft over the Persian Gulf near Kuwait.

In some cases, military aviation accidents have been spectacles in full public view. In 1996, Lt. Cmdr. John Stacy Bates’ F-14A fighter plunged into a Nashville, Tenn., suburb, killing him and four other people.

In other cases, the accidents draw great scrutiny. In October 1994, one of the Navy’s first female fighter pilots, Lt. Kara S. Hultgreen, died in a training accident off Southern California, rekindling tensions within the military then over the decision to expand some combat roles for women.

The Navy spent about $100,000 to recover Hultgreen’s aircraft, which was found in water about 4,000 feet deep. Investigators concluded that the accident had resulted from a combination of pilot error and mechanical failure.

The collision that killed Poloski did not seize any headlines, but it cast a spotlight on the elite Top Gun fighter pilot fraternity — roughly 560 carrier-based pilots — and their operations.

Born in San Diego, Poloski grew up near the Miramar Naval Air Station. His mother started taking him to Blue Angels air shows before he could walk, inspiring his lifelong enthusiasm for planes.

Growing up, he could not decide whether to become a fighter pilot or a doctor, his mother, Miriam Kendrick, said in a telephone interview. He considered attending medical school after some years in the cockpit, she said.

Like a lot of fighter pilots, Poloski loved fast cars and expensive motorcycles. A garrulous Southern Californian, he ran marathons, surfed and snowboarded. But flying was his passion.

“He loved the speed, the thrill, the fact very few people do it,” Kendrick said. “He loved everything about it.”

In April 2014, Poloski became a member of Strike Fighter Squadron 94, the Mighty Shrikes, based in Lemoore, Calif.

Soon after leaving its home port in San Diego, the crew of the Carl Vinson learned that it would join the fight against the Islamic State in the fall. Poloski was Jewish and a student of the Holocaust, and reports of the militants beheading hostages, including Americans, and slaughtering Shiite Muslims because of their faith stirred something inside the young aviator, his mother said.

“He told me, ‘I’m going to take out those bastards,’” his mother said. He never got the chance.

At 5:37 p.m. on Sept. 12, Poloski’s jet roared off the Vinson’s flight deck on a practice bombing mission more than 100 miles out at sea. A junior pilot, he had just over 221 flight hours in the Hornet. One minute earlier, an F/A-18C piloted by a Navy commander from another squadron with more than 2,100 hours in the jet had flown off the same deck to test repairs that had been made on the plane.

According to the accident report, Poloski accelerated to more than 350 mph, climbing to an altitude of 7,000 feet. About 10 miles from the carrier, he turned west and slowed to about 300 mph. He was apparently unaware that the commander, whose name was redacted from the final report for privacy reasons, had flown a similar route.

Around 5:40 p.m., the two planes collided. The commander told investigators he was unaware of the collision, feeling only a “thump” on his left engine. Within seconds, his plane was aflame and spinning wildly out of control. He bailed out and was later plucked from the sea.

Witnesses aboard the Vinson said they saw two fiery objects falling from the sky. Both planes immediately sank into the sea. Two oil slicks quickly formed on the surface; each plane was carrying more than 2,000 gallons of jet fuel.

The search for Poloski over the next 36 hours covered more than 3,000 square miles and involved several ships and aircraft as well as satellite imagery. His recovered helmet “was damaged, including a crack that extended from the bottom right side to the crown with one hole halfway up the crack,” the report said.

Investigators determined that Poloski’s plane “came underneath the bottom left rear” of the other fighter jet. Based on that and the helmet, investigators surmised that he had died of “massive head trauma.”

There were no onboard devices to warn the pilots of the impending collision. Controllers on the carrier were focusing on other jets landing. Investigators addressed the lack of situational awareness. “An overreliance on technology can be a disadvantage,” the report said.

Investigators also calculated the cost of the lieutenant’s lost plane: $77,343,287.80.

Six days after the crash, sailors and pilots gathered at the Vinson’s hangar deck for a memorial. Pilots flew a missing man formation — with one jet absent — over the carrier at the end of the ceremony. The Blue Angels, who also fly F/A-18Cs, the Corvette of the skies, later dedicated one of their shows to Poloski.

“He persistently displayed his contagious enthusiasm for our unique profession,” Cmdr. Michael Langbehn, Poloski’s squadron commander, said in an email. “He was determined to do what he must to be the best among us.”