The Serious Fraud Office has visited the offices of Sanjeev Gupta's Liberty Steel, demanding information.
The SFO launched an investigation into suspected fraud and money laundering by parent firm GFG Alliance last May.
GFG owns a collection of businesses in energy, steel and trading, employing about 35,000 people, including thousands of staff in the UK.
Mr Gupta, its executive chairman, came under scrutiny after GFG's main lender Greensill Capital collapsed.
The finance company went under after its insurer refused to renew cover for the loans it was making. It was the main lender to GFG's Liberty Steel, which employs 3,000 people in England, Scotland and Wales.
Once described as the "saviour of steel", SFO officers visited addresses linked with Mr Gupta's operations on Wednesday and requested documents including company balance sheets and annual reports.
The SFO could not provide further comment on the investigation as it is still ongoing.
But it did confirm that its teams spoke with executives at offices across Britain, who co-operated with the operation.
The SFO opened its investigation into GFG last May over suspected fraudulent trading and money laundering, including its financing arrangements with Lex Greensill's company.
GFG used so-called supply chain finance services offered by Greensill. This meant that if GFG sold a product to a different company, it could send the invoice to Greensill and be paid right away, rather than having to wait potentially months for the customer to pay its bills.
In a letter seen by the BBC, Liberty Steel staff were told: "Since then we have consistently rejected any wrongdoing on our part and pledged to co-operate fully to ensure they can conclude their investigations as quickly as possible."
It also described the site visits today as "expected" and that it would comply with requests for information and "co-operate fully in any manner".
"Please rest assured that this does not impact the operation of our companies and we must continue to focus on our business plans and operating safely," its chief transformation officer, Jeff Kabel, wrote.
According to the Financial Times, French prosecutors also visited GFG's Paris office and an aluminium smelter in Dunkirk earlier this week, where they questioned executives.
British MPs have previously raised a series of concerns over Sanjeev Gupta's leadership of GFG Alliance, including the unusual funding and company structures, as well as a series of accounting "red flags".
Politicians also recommended that the Insolvency Service should consider whether Mr Gupta has breached his fiduciary duties as a company director.