AmEx Is a Bank, But Won't Act Like One

"I don't necessarily believe that every financial institution needs to be a branch-driven bank," he says. "This is really about deposits."

American Express said in a statement that there would be no fundamental changes to its strategy.

"We want to be best positioned to take advantage of the various programs the federal government has introduced or may introduce to support U.S. financial institutions," CEO Ken Chenault said in a statement. "We will continue to build a larger deposit base to broaden our funding sources.

The company already has two bank subsidiaries: American Express Centurion Bank, an industrial loan company chartered in Utah, and American Express Bank FSB, a federal savings bank. The company issues proprietary credit and charge cards, funds card member loans and offers certificates of deposit.

Maurer says it's possible that American Express may purchase a bank if there is "an opportunity to buy a modest size deposit franchise." But he doesn't expect the company to do any large acquisitions.

FBR's Valentin says that competition for deposits among banks remains intense and that because American Express lacks the branch presence to be competitive it "will remain reliant upon high-cost CDs and, potentially, money market accounts," he writes.

"While in past conversations, American Express has downplayed its intentions of becoming a depository institution, [Monday's] surprising announcement likely is a reflection of the state of the [asset-backed securities] market. American Express is disadvantaged more than its peers, in our opinion, given its larger reliance on capital markets for funding," Valentin writes.