The Fed had already been active in the market for short-lived loans between banks and financial institutions — called the repurchase, or “repo,” market — for months, starting after rates in that obscure but important corner of the financial system’s plumbing spiked back in September. It had recently been shrinking the size of its injections as markets calmed.

But demand at its regular repo operations surged as markets swung, fueling speculation by investors that the Fed might lift the size of its offerings.

Officials have also been buying $60 billion in short-term Treasury bills each month to build up the financial system’s buffer of bank reserves, essentially deposits at the Fed. The goal was to keep cash flowing smoothly so that borrowing costs in money markets would stay under control.

“We will ensure that the supply of reserves in the banking system remains ample,” John C. Williams, the New York Fed president, said in a speech last week. “We are monitoring conditions in money markets closely.”

Many economists expect the Fed to do more to protect growth while keeping financial plumbing functioning normally.

The Fed is widely expected to slash rates by another half point by March 18, the conclusion of its next meeting. Many investors anticipate that the Fed, which cut rates to near zero during the financial crisis, will return to that level by April.

And when it comes to market functioning, “in our view much more will need to be forthcoming and very soon,” economists at Evercore ISI wrote in a note on Monday. They speculated that the central bank might extend its Treasury bill buying program, which would help to keep the financial system flush with cash and potentially help to avert short-term funding disruptions.