The last day of 14-year-old Raja Hamid Yahya al-Oud’s life began like any other.

She got up early along with the rest of the family because there was always a lot of work to do on the farm in the spring planting season. White drones had intermittently circled above their cornfields for the last few weeks, but there was no sign of them that morning.

Raja and her mother, Amira, liked to take breaks under the acacia trees about 200 metres from the house. At 4pm, this was to become her final resting place.

The plane was flying too high for them to hear it coming but Amira said the sound the CBU-52 B/B cluster bomb made as it rained 220 deadly submunitions on their heads will stay with her forever. Some exploded on impact while others, still armed, fell into the fields.

Play Video 2:49 Yemeni father at the spot his daughter died: ‘she thought she would be safe here’ – video

Screaming for her daughter, Amira saw Raja’s twisted body under a small tree. Her jaw and entire right side had been ripped apart and blood had already dyed the sand around her black. A neighbour, Hamoud Mohammed al-Ghar, dodged the remaining submunitions to pick Raja up and then ran with her to the nearest road to find a car to take her to hospital. “I knew the girl was already dead,” he said. “But I had to try.” The drones circled overhead the entire time.

Raja died on 23 March 2018. But the story of the bomb that is believed to have killed her stretches back decades earlier. Using the serial number and other technical information on the CBU-52 B/B’s outer shell, the Guardian traced the munition from its manufacture in Milan, Tennessee, to Sahar farms, in north Yemen, where it ended a child’s life.

Cluster munitions were invented by the Nazis and had become a standard weapon of war across the world by the 1970s, designed to kill as many people as possible.

Facebook Twitter Pinterest Hamoud Mohammed al-Ghar on 14 September 2019, near the spot where he found the body of Raja Hamid Yahya al-Oud.

They usually consist of a hollow shell filled with hundreds of submunitions that disperse over an area as big as several football pitches as the shell falls through the air. They either explode on impact or are triggered when moved or stepped on, firing hundreds of fragments of metal that travel at the speed of bullets.

The submunitions remain a threat for decades once deployed. Those from older designs often resemble shiny silver orbs around the size of a tennis ball, making unexploded ordnance especially attractive to curious children: hundreds of people worldwide die each year after coming into contact with unexploded cluster bombs, with about 300 casualties a year in Vietnam alone.

In 2008 an international treaty called the convention on cluster munitions severely limiting the use, transfer and stockpiling of cluster bombs was adopted by 30 countries. By 2018 it had been signed by 120 states. The US, which sells arms to Saudi-led forces fighting in Yemen, was not one of them.

“Cluster munitions went through a proportionality test to measure military advantages gained versus civilian harm of their use,” said Rawan Shaif, the lead Yemen researcher at the open source investigative organisation Bellingcat, of the Geneva conventions relating to warfare. “There’s no military advantage in using a cluster munition in a farm, unless your aim is to make that area uninhabitable for generations of civilians and military alike.”

The cluster bomb that killed Raja was manufactured at the Milan Army Ammunition Plant in 1977. The large site, just north of Jackson in west Tennessee, encompasses 231 miles (372km) of roads and 88 miles (142km) of railways and is nicknamed Bullet Town by residents.

Milan’s fortunes have risen and fallen with that of the plant: work there has ebbed and flowed with America’s wars. Most of its ordnance and mortar production lines have been moved to Iowa by American Ordnance, the private contractor operating the plant. In 2013 it employed just 113 people, down from a peak of 10,000 during the second world war. By 1977 thousands of cluster bombs like the one that killed Raja were rolling off the production line.

The US no longer manufactures CBU-52s: it has long since upgraded to “smart” computer-guided cluster bombs with supposedly more accurate arming and targeting systems. One possible reason a 1977 bomb found its way to Yemen’s Sahar farms in 2018 is because the US has such a large stockpile of the weapons.

While other countries were adopting the international convention on cluster munitions in 2008, the Pentagon defended their use as a “military necessity”. It conceded to international criticism, however, by instituting a policy to reduce the failure rate of the munitions – ie the amount of unexploded ordnance left behind – to 1% or less after 2018, although that policy was actually scrapped by Donald Trump 2017.

In the interim, a good way of reducing the stockpile of outdated weapons – and making some money – was to sell them. Data from 2017 showed the US still had some 2.2m cluster bombs in its arsenal.

“Many nations, including the US, Germany, and Switzerland, have taken steps in recent years to bolster both their pre-export due diligence, and their post-delivery verification methods,” said Nic Jenzen-Jones, the director of Armament Research Services, a specialist technical intelligence consultancy. “Nonetheless, there are a lot of arms that have been exported prior to these changes that muddy the waters in terms of how researchers can track and trace items of interest.”

The US has been selling weapons, planes, tanks and ordnance to Saudi Arabia and permitting its arms manufacturers to export for decades: it has licensed $138bn (£112bn) to the kingdom in the last 10 years alone, according to research by US thinktank the Center for International Policy. After Riyadh launched its air campaign against Yemen’s Iran-backed Houthi rebels in March 2015, officials under Barack Obama worried that sales could amount to complicity in war crimes under international laws that could name the US as a co-belligerent. In 2016, they went ahead with a $1.3bn sale anyway.

Bomb sales continue: Donald Trump, who has made Saudi Arabia the cornerstone of his Middle East policy, overrode objections from Congress in March to complete the sale of more than $8bn worth of weapons to Riyadh, its coalition partner the United Arab Emirates, and Jordan.

Facebook Twitter Pinterest US-made ammunition fragments found in various bombed locations around Yemen, seen at the HQ of Yemac on 12 September 2019.

The total extent of export licences to the kingdom is not known, and neither is it known when the bomb that killed Raja was sold. The SIPRI Arms Transfers Database, the world’s most extensive record, yielded no results: it contains just a fraction of true sales, as pre-digital record-keeping was poor and states cite national security concerns to avoid disclosing records.

It is possible the CBU could have been sold by the US years ago to a third party state and then sold on again to Saudi Arabia: cluster bomb sales to Riyadh were officially stopped in 2016. The US Department of Defense and the state department did not respond to requests for information. Such inquiries are usually met with no comment.

Washington says US supplies of “smart weapons” are helping the coalition reduce civilian casualties – but as Raja’s death shows, old US-made dumb bombs are still being used.

What we do know is Sahar farms was bombed on 23 March last year after the area had been surveyed by drones for weeks. “We are just farmers,” Hamid Yahya al-Oud, Raja’s father, said over tea in his living room, surrounded by his six remaining children. “Look outside. There’s our well, there’s our goats. Why would there be Houthis here?”

At the time of publication, the coalition had not responded to the Guardian’s requests for comment.

Sahar farms lies in the countryside of Saada in north Yemen, the Houthi heartland on Saudi Arabia’s border. The whole province is littered with bombed-out buildings, bridges, roads and unexploded ordnance. The green cigar-like shell of the CBU-52 B/B cluster bomb is one of the most common of those dropped on the area, according to Abdullah Umayyad, the manager of the UN-funded Yemen Executive Mine Action Centre (Yemac) field office in Saada.

The coalition rarely acknowledges civilian casualties from airstrikes, making justice, compensation or even simple answers for affected families about why they were targeted almost impossible.

“It took two days to clear out Sahar farms after that strike,” said Umayyad. “The work is very dangerous. Three men have been killed in clearance operations here in Saada. My old boss lost his hands, eyes and nose after trying to disarm a cluster bomb submunition last year.”

Despite condemnation from human rights groups over airstrikes in Yemen, Yemac’s records suggest more cluster bombs are being dropped on Saada than ever: at least 147 cluster munitions were recovered in the province in the first six months of 2019, up on 122 in the same period of 2018.

Facebook Twitter Pinterest More American-made ammunition fragments found in Yemen.

Its record-keeping, however, is spotty and individual items are not catalogued. Questions of who, what and why when it comes to bombings fall outside its remit: the political fallout could impact the team’s ability to work across different parts of the war-torn country.

US-made bombs continue to rain down on north Yemen with almost total impunity.

At Sahar farms, the drones have been back in the last two months. “They already killed my daughter,” said Hamid. “Now they’re back for more.”