He explained that AIG lent out high-grade securities like treasuries in exchange for cash. Normally it would have been a safe business, but because the company had invested that cash in subprime mortgages that had lost enormous value, no one could peg their exact price, which made them nearly impossible to sell. If AIG’s counterparties—the firms on the other side of the trade—should all demand their cash back at the same time, Willumstad said, he could run into a serious problem.