The US Department of Justice has started investigating a data provider and dealers in the credit derivatives market for potential violations of the US Sherman Act, which prohibits abuses of monopoly power or other forms of collusion.

Demands were sent to more than a dozen dealers for several years’ of detailed information about trading and pricing, according to people who received the letter. The DoJ letter was sent to banks with an equity stake in Markit Group, which provides pricing data on markets including the credit default swaps (CDS).