The Securities and Exchange Commission has stepped up its probe of asset valuation at hedge fund D.B. Zwirn & Co. with a round of subpoenas.

The regulator, which is looking into alleged irregularities about how D.B. Zwirn valued its high-yield bond portfolio, is seeking information from investors and staff, the Financial Times reports. In addition to questions about valuation, the SEC is also asking about internal fund transfers and the billing of expenses to investors.

The New York-based firm, which is shuttering its largest funds and returning $4 billion of the $5 billion it manages, last year told investors that an internal audit turned up improper financial transfers and accounting of expenses. D.B. Zwirn also said that one of its fund managers did not “follow a systematic pricing methodology” for high-yield bonds, and returned $818,398 in management fees earned on the discrepancy.

The manager in question left the firm in 2005.