For months, the nation’s largest banks have struggled to regain investors’ trust. In the center of the vortex is Citigroup, whose precipitous stock-market plunge accelerated on Thursday, sending shock waves through the financial world.

The shares slumped 26 percent Thursday; the bank has lost half its value in just four days. The chief executive, Vikram S. Pandit, will hold a meeting for senior managers Friday to update them on the bank’s condition.

Investors and analysts have long pressured the bank to consider ways to lift its stock price, including splitting the company or selling pieces. While a few also say the company should consider selling itself outright, there is no certainty that any change would happen soon. Senior executives say the company is financially strong and has ample financing options. Moreover, there are few buyers who would be willing to pay a price that Citigroup would want for its most valuable assets.

Citigroup executives are seeking to stabilize the stock price, but at this point they are not actively exploring selling or splitting up the company, according to two people with direct knowledge of the discussions.