White House economists have identified a potential stumbling block to maintaining the U.S. economy’s momentum: a lack of well-trained workers.

The economy appears poised to expand this year at the fastest clip since the recession ended in 2009. That has allowed employers to extend their streak of consistent hiring, which began in 2010, and push the unemployment rate to nearly its lowest level in 50 years.

But there also is a growing gap between the rising number of job openings and the number of workers equipped to fill them, and this could limit growth in the long run, according to a paper from President Donald Trump’s Council of Economic Advisers released Tuesday.

One problem is the overall size of the pool. A smaller share of adults between 25 and 54 years old are working or looking for jobs than before the recession began in 2007.

Another problem, the economists wrote, is many Americans on the sidelines of the labor market need more education or other skills to fill the types of jobs that are being created. The paper found a disproportionate share of those not seeking or holding jobs don’t have college degrees, an indication they don’t have the advanced skills needed to fill many job openings.

A lack of skills, however, is only one reason many Americans are out of the workforce. Other economists have pointed to other reasons, including the rising cost of child care, the need to care for elderly parents and the opioid crisis.

Read more in the Wall Street Journal.