WASHINGTON — The Federal Reserve will begin shrinking the enormous portfolio of bonds it amassed after the 2008 financial crisis, trying to sustain the economy.

The move reflects a strengthened economy and could mean higher rates on mortgages and other loans over time.

The Fed announced Wednesday that it will let a small portion of its $4.5 trillion balance sheet mature without being replaced, starting in October with reductions of $10 billion a month and gradually rising over the next year to $50 billion a month.

The central bank said it’s leaving its key short-term rate unchanged but hinted at one more rate hike this year if persistently low inflation rebounds.

The Fed policymakers’ updated economic forecasts show an expectation for three more rate hikes in 2018.

The Fed has telegraphed its move for months. The risk exists that investors could be spooked by the rising number of bonds being transferred back into private hands.

If that happens, long-term rates might surge undesirably high, weighing on the economy.

Any damage in the markets could extend to other assets, such as stocks, which have set record highs as investors have shifted money into stocks and away from low-interest bonds. There is concern, too, that rates could climb faster if other central banks follow the Fed’s lead and reduce their own bond holdings.

The Fed’s plan for shrinking its balance sheet is so gradual that the total would remain above $3 trillion until late 2019. Some economists say they think the figure could end up around $2.5 trillion, still far above the $900 billion the Fed held in its portfolio in pre-crisis days.