The business editor is Simon Jack.
Russia is stealing $600m worth of steel from plants and ports in Ukranian.
The last holdout of Ukrainian soldiers and civilians was owned by the firm.
Some of the steel was bound for customers in the UK, according to the chief executive.
The Kremlin didn't say anything.
After almost three months of relentless assault, Mariupol fell to Russia in May.
300 employees and 200 employees' relatives were killed in the assault on the Azovstal plant, which accounted for 40% of all Ukrainian steel production.
European customers paid for thousands of tons of steel.
According to public sources and the company's own sources, the steel was being sold in internal markets or to countries in Africa and Asia.
They're doing a lot of things. Some of the products that are already owned by the European customers are being stolen. They are not only stealing from us, but they are also stealing from the Europeans.
He said that the company would be taking future legal action and that it was documenting as much of the theft as possible.
The Russians will be facing both the international courts and the criminal courts in the future. We will be after them with everything we have.
The Russian Ministry of Defence has not responded to accusations made by the British Broadcasting Corporation.
Prof Milanovich, an expert on international law at the University of Reading, says there are a number of options to pursue a case.
It's a very difficult process and whilst it's common in conflict, suing the state and getting compensation is very rare, he said.
It would be a big deal if they labelled Russia as a lawbreaker.
Last month, the Reality Check team tracked where Russia is taking grain stolen from Ukraine after mounting evidence that Russian forces in occupied areas have been shipping produce out of the country.
Metinvest has become a part-military, part-humanitarian organization. The importance of steel workers to the Ukrainian war effort was said to have been comparable to that of soldiers.
We provide steel for armored vehicles, and we make bulletproof vests as well. The people working in the steel mills are very important to the victory of the Ukrainians.
He said he was confident the day would come when the plant would be a Ukrainian facility.
I'm optimistic. It is just a matter of time when Russia realises that they have lost the war. The Russians don't require Mariupol. Even if they stop now, Ukraine will take it back. It's going to happen soon.
Wouldn't that require a big military offensive by the Ukrainians?
When Germany and East Germany came back together, was it a big deal? There are a number of ways in which it can be returned. A big military offensive is one of the ways that Russia will have to leave because of internal issues. There is a third way that will happen in the future.
One of the defining moments of the war was the siege of Azovstal.
The Ukrainian steel industry has suffered. The people of Mariupol have suffered a terrible loss. It will be remembered as an act of heroism for the military, who were protecting the mill for so long. It will be remembered as a sign of the strength of a group of people. It will be remembered that way.