JPMorgan's holdings of home-loan bonds from outside the U.S. soared 35-fold in the past three years. Now, with its chief investment office facing scrutiny after a $2 billion trading loss, investors are raising concern the European market’s biggest buyer will pull back.

The largest U.S. bank by assets accelerated its purchases last quarter, adding $8.5 billion to lift its total to $74.5 billion, according to regulatory filings. The New York-based company’s investments approached 9 percent of the size of the Dutch and U.K. mortgage-bond markets it’s been focusing on.

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