NEW YORK—Wall Street extended its slump into yet another week Monday as investors worried that even a safety net set up for mortgage financiers Fannie Mae and Freddie Mac wont’ head off further troubles in the financial markets.

Investors’ latest unease about the banking sector comes in a week when many financial names are to issue quarterly reports—many of which will likely include sizable write-downs of souring mortgage debt.

The Treasury and the Federal Reserve said Sunday they would aid Fannie Mae and Freddie Mac if needed. Wall Street has been on edge about the wel...