Index funds have helped Vanguard become a $3 trillion asset manager for the first time, but some investors may be surprised to learn exactly which kind of index funds are accounting for a significant portion of the firm’s growth.

Pennsylvania-based Vanguard is within striking distance of taking over the number two spot among U.S. providers of exchange traded funds and the company has been the most impressive asset gatherer among ETF providers this year, though it ceded that title to iShares in the second quarter. [iShares Leads Q2 ETF Flows]

However, passively managed index mutual funds have led Vanguard’s asset growth this year, not ETFs.

“The index fund vs. ETF trend continued this year, with investors pouring $65.7 billion into Vanguard’s index mutual funds, compared with $37.1 billion into index ETFs in the seven months through July,” reports Kirsten Grind for the Wall Street Journal, citing Morningstar data.

That extends last year’s trend when investors poured $99.8 billion into Vanguard’s index-based mutual funds compared to $55 billion into comparable ETFs with $24 billion of outflows from actively managed mutual funds, according to the Journal.

Vanguard currently does not offer actively managed ETFs.