On Thursday last week, Saudi Arabian fighter planes bombed a Yemeni school bus in a targeted attack that killed more than 50 people, mostly children.
Two days later, Yemeni journalist Nasser Arrabyee tweeted photos from the scene of attack that showed US-made bombs were used by the Saudi air forces.
Rights groups have been criticising the US for massive weapons sales to Saudi Arabia, which is engaged in a bloody military conflict with neighbouring Yemen.
The pictures tweeted by the journalist showed bomb parts that looked like they were from the deadly MK-82, a 500-pound bomb made by American company General Dynamics.
The MK-82 bombs were used earlier as well during Saudi Arabia's three-year war with Yemen, which has so far killed more than 10,000 people.
In 2016, a Saudi attack on a funeral procession in Yemeni capital Sana'a, using the Raytheon Mark 82 general-purpose free-fall bomb, had killed 140 people.
Ever since Saudi Arabia's Mohammad-bin Salman-led war on Yemen started, rights groups from around the worked have pressed the US not to sell weapons to the country. However, after President Donald Trump came to power, the US announced a fresh round of weapons sale to Riyadh worth $110 billion.
Thursday's attack in the Dahyan area of the Houthi-controlled Saada province had also injured nearly 80 people.
Amid criticism that the US bombs were used to kill innocent children, the Pentagon clarified that it would be difficult to say where the bomb used the attack came from.
"we may never know if the munition [used] was one that the U.S. sold to them. We don't have a lot of people on the ground," US Central Command spokesman Maj. Josh Jacques said, according to Gulf site Albawaba.
Russian portal RT reported that despite pressures from rights groups, the US awarded Lockheed Martin/General Dynamics "key contracts to supply the MK-82 500-pound bombs to the Arab coalition' in 2017. The report also said that the US had approved the sale of MK-82 to Saudi Arabia, United Arab Emirates, France, and Iraq in 2016.
How could the Pentagon now say it doesn't know where the bombs came from the report asked.