WASHINGTON — Risky lending caused private student loan debt to balloon in the past decade, leaving many Americans struggling to pay off loans they can’t afford, a study says.

Private lenders gave out money without considering whether borrowers would repay, then bundled and resold the loans to investors to avoid losing money when students defaulted, according to the study, released today.

Those practices are closely associated with subprime mortgage lending, which inflated the housing bubble and helped bring about the 2008 financial crisis.

Education Secretary Arne Duncan said the government must do more to ensure that people who received private loans enjoy the same protections as those who borrow from the federal government.

Student loans fall into two main categories: Loans directly from the government and those offered by banks and other private financial companies. The report focused on private student loans, which spiked from $5 billion in loans originated in 2001 to more than $20 billion in 2008. After the financial crisis, as lending standards tightened, the market shrank to $6 billion in 2011.

American consumers still owe more than $150 billion in private student loan debt, the study said. Including federal loans, Americans now owe more than $1 trillion in student loan debt, the study said.