The Federal Reserve bought $1.31 billion worth of corporate-bond exchange-traded funds leading up to May 19, spreading those purchases across several investment-grade and high-yield products.

Aimed at continuing to stabilize corporate bond markets after they came under stress in March, the Fed's ETF purchases kicked off May 12.

In all, the Fed bought shares of 15 corporate bond ETFs — the majority of which tracked investment-grade-rated debt — through trades that were settled as of May 19, according to a report it provided to Congress that detailed disclosures on each transaction for the first time. The central bank has continued to purchase corporate bond ETFs since, with its Secondary Market Corporate Credit Facility holding roughly $2.98 billion in ETFs as of May 27, according to the Fed's weekly balance sheet update.

BlackRock Inc.'s iShares iBoxx U.S. Dollar Investment Grade Corporate Bond ETF, known by its ticker LQD, was the most heavily purchased fund by the Fed during the time frame. The central bank bought 2,521,892 shares of the ETF, carrying a market value of $326.3 million as of May 19. Its next-biggest purchases were in Vanguard Group Inc.'s Intermediate-Term Corporate Bond ETF and Short-Term Corporate Bond ETF.

The Fed has bought ETFs sponsored by five asset managers: BlackRock, Vanguard, State Street Global Advisors Inc., Van Eck Associates Corp. and DBX Advisors LLC. Eight of the 15 ETFs purchased by the Fed were products of BlackRock, which the Fed brought on as its initial investment manager to facilitate the corporate bond facilities. The world's largest asset manager has said it will not charge any asset management fees for funds held under the program.

For now, the Fed is buying corporate bond ETFs only from the primary dealers its New York Fed trading desk usually transacts with. The three biggest sellers to the Fed through that time period were BofA Securities Inc., Morgan Stanley & Co. LLC and Barclays Capital Inc.

The central bank is also planning to purchase eligible underlying corporate bonds themselves, although those efforts have not yet started. The Fed is limiting its primary and secondary corporate bond purchases to investment-grade companies and certain "fallen angels" that met that criteria as of March 22 but were recently downgraded.

The ETF purchases, however, include purchases of funds that have "junk" bonds as underlying securities. The Fed's biggest high-yield purchase was in HYG and JNK, BlackRock and State Street products that the central bank respectively bought 1,255,084 shares and 905,284 shares of during the period.