Amnesty International on Wednesday called for an investigation into a US airstrike on a migrant detention center in Sa’ada, north-western Yemen, in April.

The airstrike was part of “Operation Rough Rider”—a 45-day campaign in which the US struck Houthi rebel targets in Yemen, in response tp Houthi missiles attacks on ships in the Red Sea. The US strike resulted in a high number of civilian causalities.

Amnesty investigation found that there “was no evidence that the migrant detention center was a military objective.” Survivors stated that the migrant detention center was an “open space” where there was visibility of the occupants of the building.

“The US must conduct a prompt, thorough, independent, impartial, and transparent investigation into the air strike on the Sa’ada migrant detention center and make the results public.” said Kristine Beckerle, Amnesty International’s deputy regional director for the Middle East and North Africa.  

In March, US President Donald Trump stated that Houthi attacks have caused a “sustained negative impact on global trade and economic security of the United States.” As the number of merchant ships in the Red Sea dropped, Trump asserted that “75% of US- and UK-affiliated vessels rerouted around Africa instead of transiting the Red Sea.” 

In July, Human Rights Watch condemned Houthi attacks on commercial ships on the Red Sea as war crimes. The EU stated attacks by the Houthi in the Red Sea endangered the lives of crew members and destabilized global commerce.

Amnesty previously condemned the US airstrikes on April 28, stating that they violated international law by failing to determine if attack targets fulfill legitimate military objectives.