By the middle of March 2020 a sense of anxiety pervaded the Federal Reserve. The fast-unfolding coronavirus pandemic was rippling through global markets in dangerous ways.

Trading in Treasurys — the government securities that are considered among the safest assets in the world, and the bedrock of the entire bond market — had become disjointed as panicked investors tried to sell everything they owned to raise cash. Buyers were scarce. The Treasury market had never broken down so badly, even in the depths of the 2008 financial crisis.

The Fed called an emergency meeting on March 15, a Sunday. Lorie Logan, who oversees the Federal Reserve Bank of New York’s asset portfolio, summarized the brewing crisis. She and her colleagues dialed into a conference from the fortresslike New York Fed headquarters, unable to travel to Washington given the meeting’s impromptu nature and the spreading virus. Regional bank presidents assembled across America stared back from the monitor. Washington-based governors were arrayed in a socially distanced ring around the Fed Board’s mahogany table.

Ms. Logan delivered a blunt assessment: While the Fed had been buying government-backed bonds the week before to soothe the volatile Treasury market, market contacts said it hadn’t been enough. To fix things, the Fed might need to buy much more. And fast.

announced an enormous purchase program, promising to make $500 billion in government bond purchases and to buy $200 billion in mortgage-backed debt.

It wasn’t the central bank’s first effort to stop the unfolding disaster, nor would it be the last. But it was a clear signal that the 2020 meltdown echoed the 2008 crisis in seriousness and complexity. Where the housing crisis and ensuing crash took years to unfold, the coronavirus panic had struck in weeks.

As March wore on, each hour incubating a new calamity, policymakers were forced to cross boundaries, break precedents and make new uses of the U.S. government’s vast powers to save domestic markets, keep cash flowing abroad and prevent a full-blown financial crisis from compounding a public health tragedy.

The rescue worked, so it is easy to forget the peril America’s investors and businesses faced a year ago. But the systemwide weaknesses that were exposed last March remain, and are now under the microscope of Washington policymakers.

cut interest rates to about 1 percent — its first emergency move since the 2008 financial crisis. Some analysts chided the Fed for overreacting, and others asked an obvious question: What could the Fed realistically do in the face of a public health threat?

“We do recognize that a rate cut will not reduce the rate of infection, it won’t fix a broken supply chain,” Chair Jerome H. Powell said at a news conference, explaining that the Fed was doing what it could to keep credit cheap and available.

But the health disaster was quickly metastasizing into a market crisis.

Lockdowns in Italy deepened during the second week of March, and oil prices plummeted as a price war raged, sending tremors across stock, currency and commodity markets. Then, something weird started to happen: Instead of snapping up Treasury bonds, arguably the world’s safest investment, investors began trying to sell them.

The yield on 10-year Treasury debt — which usually drops when investors seek safe harbor — started to rise on March 10, suggesting investors didn’t want safe assets. They wanted cold, hard cash, and they were trying to sell anything and everything to get it.

officially declared the virus outbreak a pandemic, and the morning on which it was becoming clear that a sell-off had spiraled into a panic.

The Fed began to roll out measure after measure in a bid to soothe conditions, first offering huge temporary infusions of cash to banks, then accelerating plans to buy Treasury bonds as that market swung out of whack.

But by Friday, March 13, government bond markets were just one of many problems.

Investors had been pulling their cash from prime money market mutual funds, where they park it to earn a slightly higher return, for days. But those outflows began to accelerate, prompting the funds themselves to pull back sharply from short-term corporate debt markets as they raced to return money to investors. Banks that serve as market conduits were less willing than usual to buy and hold new securities, even just temporarily. That made it harder to sell everything, be it a company bond or Treasury debt.

The Fed’s announcement after its March 15 emergency meeting — that it would slash rates and buy bonds in the most critical markets — was an attempt to get things under control.

But Mr. Powell worried that the fix would fall short as short- and long-term debt of all kinds became hard to sell. He approached Andreas Lehnert, director of the Fed’s financial stability division, in the Washington boardroom after the meeting and asked him to prepare emergency lending programs, which the central bank had used in 2008 to serve as a support system to unraveling markets.