UBS Group has revealed that it was compelled to engage in an unwanted buying of its cross-town rival Credit Suisse Group amid a global banking crisis that deteriorated the latter’s finances, prompting authorities to take swift action.

In a Tuesday filing to the U.S. Securities and Exchange Commission (SEC), UBS told investors it had less than four days to conduct due diligence given the "emergency circumstances."

It estimated a hit of about $17 billion from the takeover.

Switzerland’s biggest bank agreed to buy its smaller rival after the latter had endured a challenging year.

Credit Suisse’s involvement in corporate collapses spooked clients, who began withdrawing their money. This trend accelerated when U.S. bank failures sparked fear of a broader banking crisis.

The wave of deposit outflows and a major share-price drop prompted Switzerland’s central bank on March 15 to offer Credit Suisse liquidity assistance.

The next day, UBS and Credit Suisse signed a confidentiality agreement upon which the former began due diligence, the UBS filing showed.

On March 19, the Swiss National Bank announced that UBS would buy Credit Suisse for 3 billion Swiss francs ($3.4 billion) in stock and assume a loss of as much as 5 billion francs from winding down part of the business.

The filing showed that the final price was raised from 1 billion francs.

Interest from UBS in buying Credit Suisse began in October when the ad hoc Strategy Committee of its board of directors reviewed its rival’s distressed situation, according to the filing.

By then, Credit Suisse was experiencing deposit and net asset outflows at levels substantially exceeding rates of the July-September quarter, UBS said.

In early December, UBS management undertook a preliminary assessment of the consequences of a Credit Suisse purchase, which it presented to the Strategy Committee on Dec. 19.

In February, the Strategy Committee and Board of Directors concluded that an acquisition was "undesirable." They recommended further analysis to prepare for a scenario in which Credit Suisse was so complicated that regulators could ask UBS to step in.

UBS said it carried out financial analyses from January to mid-March and assessed potential legal structures and possible measures to address concerns and any negative impact on itself in case authorities proposed an acquisition.

From December to mid-January, Credit Suisse executives had also been discussing its options with the government, including a merger with UBS, the UBS filing showed.