WASHINGTON — Citigroup ran into trouble under the noses of federal regulators. But even after taxpayers rescued the financial giant, regulators failed to monitor the company adequately, according to reviews by the Federal Reserve.

Excerpts of two Fed reviews were released on Wednesday as the committee examining the causes of the financial crisis began three days of hearings on the problems at Citigroup, Fannie Mae and the subprime mortgage market.

The panel heard a strong defense of the Fed from its former chairman, Alan Greenspan, who fended off a barrage of questions about the Fed’s failure to crack down on subprime mortgages and other abusive lending practices during his tenure.

But the excerpts, culled from thousands of documents turned over to the bipartisan Financial Crisis Inquiry Commission, painted a troubling picture of the Fed’s oversight of Citigroup both before and after Mr. Greenspan left the Fed — and again after Citigroup received three taxpayer-financed bailouts.