WASHINGTON — And at the end of June, the Federal Reserve finished its work and rested.

The nation’s central bank said Wednesday that it would complete the planned purchase of $600 billion in Treasury securities next week, then pause its three-year-old economic rescue campaign, leaving in place existing aid programs, but doing nothing more, for now, to bolster growth.

At the same time, the Fed said the economy was expanding less quickly than it had predicted. It now projects a growth rate of 2.7 percent to 2.9 percent in 2011, and 3.3 percent to 3.7 percent in 2012. Both estimates are markedly below its last forecast in April.

As growth has sputtered this year, economists have pointed to higher oil prices, the Japanese earthquake, bad weather, a lack of confidence. The unifying theme was that spending and investment would surge as these temporary impediments subsided. The Fed’s latest forecast, however, reflects the surprising weight of deeper and more intractable problems, including unsustainable public and private debts, the wreckage of the housing market and trade imbalances.

Roughly 25 million Americans were unable to find full-time work in May, and the central bank projects that most of those people will remain unemployed for years to come.