It was 5 May 1945, a warm spring day, and the second world war felt very distant to Annie Fagen, aged 13, as she prepared to join friends for a picnic in the Oregon woods.

They were already up in the forest, a bucolic landscape scented with sage, scouting for a creek where they could fish and lay blankets.

Annie was delayed in Klamath Falls, a small logging town, accompanying her mother who needed new glasses. She never got to join her friends. “A group of girls came up and asked if I’d heard what had happened. They said there was an explosion, that they had been killed, but it was a military secret.”

The six picnickers who died – five children and a pregnant woman – were the only people killed by enemy action on the American continent in the second world war.

Seven decades later Annie, now a great-grandmother of 83, shook her head. “We didn’t known anything about the balloons. No one told us.”

A Japanese “fire balloon” packed with explosives had flown approximately 5,000 miles across the Pacific, landed on Gearheart mountain, and lay dormant until the victims inadvertently detonated it.

It was one of hundreds of such balloons that floated across the ocean and landed on the western seaboard, from Alaska to Mexico, and as far inland as Texas, Wyoming and Michigan. They carried incendiary devices and high explosives intended to start forest fires and sow terror.

Annie Patzke, 83, who missed out on the fatal picnic by chance, at her diner in Klamath Falls. Oregon. Photograph: Rory Carroll for the Guardian

“They were the first intercontinental weapons delivery system,” said Robert Mikesh, a retired US air force officer and military aviation historian who has written about the balloons.

US authorities hushed up the attacks and ordered a media blackout to prevent the Japanese knowing that their experimental weapons were in fact reaching the US.

The picnic massacre lifted the official veil of secrecy. But the balloon campaign, which began 70 years ago on 3 November 1944, never became widely known and to this day remains largely forgotten.

President George W Bush overlooked it after 9/11 when he told Congress that apart from Pearl Harbor Americans had known wars only on “foreign soil” for 136 years.

Japan’s fire balloons evoke baffled looks, said Mikesh. “I can sit at any dinner table and bring up the subject and it’s, ‘what are you talking about?’ People are amazed.”

To those affected by the blast on Mount Gearheart the Japanese campaign is not a historical footnote, or a terror template, but a source of enduring grief.

Archie Mitchell, a young pastor with the Christian and Missionary Alliance Church, who led the picnic, was the only survivor. The blast claimed his pregnant wife Elsie, 26, and five children from his Sunday school: Edward Engen, Jay Gifford, Sherman Shoemaker, Dick Patzke and his sister Joan Patzke, all aged between 11 and 14.

The Patzkes showed remarkable resilience and forgiveness in overcoming that and additional tragedies.

“There’s a reason I was spared but I don’t know why,” said Annie, who became a Patzke after marrying one of Dick and Joan’s older brothers after the war.

Seated in the family’s diner at Klamath Falls, Annie said her mother’s glasses saved her life. “God had a plan for me to be here still,” she said.

She still thinks of Joan. “She was my best friend. I wonder what she would be like if she had got to grow old; I wonder if we’d go deer hunting together.”

vfire balloons map Photograph: Guardian

The chain of events leading to the picnickers’ deaths dates from the 1920s when a Japanese meteorologist discovered that air currents swept eastwards at 30,000 feet. In 1944 the military sought to harness them to avenge US air raids.

Japan’s options were limited. It had lost its aircraft carriers. Submarines had briefly shelled targets in Oregon and California in 1942, causing negligible damage, but by 1944 such attacks were no longer feasible.

So the Ninth Army Technical Research Laboratory devised hydrogen balloons known as Fu-Go (wind-ship weapon) as payback. Using sandbags as ballast and an altimeter to control altitude, with bombs suspended from a ring, they took 30 to 60 hours to reach America’s coast.

“They were very cleverly designed. They would expand and rise during the day and cool and descend at night, with a barometer to keep the right range,” said Dave Tewksbury, a member of the geosciences department at Hamilton College, New York, who used mapping software to track them in a 2008 Geological Society of America project. “This was a continuous cycle of rise and sink associated with day and night as they crossed the Pacific.”

Japanese records indicate about 9,000 were launched. Several hundred are believed to have made it to the US. Most caused little or no damage, thanks partly to a wet winter dousing flames, said Mikesh, the author. “They were not a successful weapon but they were hazardous. Mischievous, for want of a better word.”

By extraordinary fluke, in March 1945 one of the balloons damaged power lines to the Hanford, Washington, nuclear facility which was producing plutonium for the bomb that later destroyed Nagasaki. Work was briefly interrupted.

The US media blackout helped keep the Japanese in the dark about what impact, if any, their efforts were having.

So the picnickers had no reason to expect or fear curious objects in the woods on the balmy afternoon of 5 May. Mitchell, the pastor, was apparently fetching food from the car when he heard one of the group say they had found something. Moments later, the explosion.

Cora Conner, 16, was manning her family-run telephone exchange in the nearest town, Bly, when a forest service official rushed in demanding a line. “He said there had been a bomb. I knew it must have been the Patzkes and the others,” said Conner, now 85. “Everyone knew everyone. They’d been in my house on the way up there.”

The fire balloon exhibit at the Klamath county museum. Photograph: Rory Carroll for the Guardian

The military barred access to the site so townspeople besieged the exchange for news. “They were all tore up. They yelled at me and shook their fists, they knew I knew something, but I was told not to say a word to anyone.” The shock deeply affected Conner. “It took me 40 years to be able to talk about this.”

Eventually the military lifted the secrecy, explained what had happened, and granted access to the crater. “Trees were shorn. The car had shrapnel,” recalled Annie.

The Japanese, hammered by air raids and sliding to defeat, had abandoned the campaign in April, a month earlier. But the Mount Gearheart balloon lay dormant, its explosives intact.

The Patzke family then suffered another blow: they learned that Jack Patzke, 21, one of Dick and Joan’s brothers, an air force sergeant, had been captured and killed in Germany, apparently while trying to escape.

Betty, the only one of the original 10 Patzke children still alive, is now a frail 93-year-old living with her daughter, Geraldine, in North Carolina.

The family has no bitterness towards the Japanese. In 1987 some of the Japanese women who helped make the balloons visited the site of the explosion to apologise. “Those young girls, they just had to do what they were told,” said Annie, who married Pat Patzke.

Conner, the former telephone exchange operator, also forgave. “We had a dedication ceremony and some dinners. It was a pretty big thing.”

The Klamath county museum and a balloon museum in Albuquerque have exhibits but most people remain unaware of the fire balloons, said Marilee Mason, curator of the Albuquerque museum. “It was one of the greatest secrets of the war. These days it would be over the internet in a minute.”

A putative campaign to ignite forest fires did in fact roil the internet: in 2012 the al-Qaida online magazine Inspire urged jihadists to plant “ember bombs” in US forests.