The Democratic Republic of Congo’s government notified Apple of concerns that the company’s supply chain may be tainted by conflict minerals sourced from the central African nation.

A group of international lawyers engaged on behalf of the Congolese State sent a list of questions to the US tech giant on April 22 and demanded answers within three weeks, according to a statement published on the website of lawyer Robert Amsterdam on Thursday. The statement coincided with the release by the law firm of a report accusing neighbouring Rwanda of laundering tin, tungsten and tantalum — known as 3T minerals — and gold from Congo.

“Although Apple has affirmed that it verifies the origins of minerals it uses to manufacture its products, those claims do not appear to be based on concrete, verifiable evidence,” Amsterdam said. “The world’s eyes are wide shut: Rwanda’s production of key 3T minerals is near zero, and yet big tech companies say their minerals are sourced in Rwanda.”

Apple, which is known for rigidly policing its sprawling supply chain, has addressed accusations surrounding conflict minerals for years. The company disclosed back in 2016 that it began fully auditing its suppliers for the use of minerals linked to violent militia groups.

The US firm, which uses the minerals in its iPhones and other devices, referred a request for comment to a March filing in which the company said it was reasonably confident that none of its suppliers of tin, tungsten, tantalum and gold financed or otherwise benefited armed groups in the region, though it did remove 14 smelters and refiners that didn’t participate in an audit.

It also pledged to source “responsibly” and work to improve working conditions among mining communities, including in Congo.

Rwanda’s government didn’t respond to a request for comment sent by text message. The East African nation is the world’s largest producer of tantalum after Congo, according to the US Geological Society. It also has deposits of tin ore and tungsten ore.

Eastern Congo has been wracked by violence since the mid-1990s in the aftermath of the Rwandan genocide. Dozens of rebel groups are active in the region, fighting over land, economic resources and ethnic disputes. The conflict has displaced at least six million people.

Rwandan President Paul Kagame has long denied that his country benefits from what Congo claims is the $1-billion a year it loses from the smuggling of gold and other minerals through its eastern neighbor. United Nations experts also accuse elements of Congo’s own army of profiting from the illicit trade in minerals from the region.

The US and European Union have laws discouraging companies from purchasing minerals linked to the conflict.

Congo government spokesman Patrick Muyaya confirmed it had hired the team of lawyers.