The Securities and Exchange Commission has stepped up its investigation into DB Zwirn, seeking information about how the $5bn hedge fund valued some of its assets, according to people familiar with the probe.

The SEC activity comes amid growing questions from investors about how hedge funds value hard-to-trade assets that do not always have market prices. The issue is of particular concern because hedge funds typically pay managers 20 per cent of investment gains each year, based on such estimated values.