U.S. companies have been bulking up on debt, introducing another wild card into financial markets already rattled by the recent tech selloff and the prospect of rising interest rates.

One slice of the high-grade corporate bond universe is fast becoming the epicenter of these concerns. There is $2.5 trillion in outstanding U.S. debt rated triple-B, according to Morgan Stanley , up from $1.3 trillion five years ago and $686 billion a decade ago. That is the most ever for companies rated triple-B, which is the lowest rung of the ratings ladder for companies that are above more speculative, or junk, bonds.

The fear: If the long economic expansion takes a turn for the worse, investors could jettison the debt of more leveraged borrowers such as triple-B issuers. That would further weigh on companies and potentially exacerbate any contraction.

The International Monetary Fund recently highlighted the triple-B risk in a report on financial stability that warned about “a buildup of financial balance sheet” debt.

“When markets start restricting access to capital in a downturn or a bear market, we tend to find that leverage levels matter a lot,” said Adam Richmond, a credit strategist at Morgan Stanley.