WASHINGTON — The investigation into President Donald Trump’s former national security adviser, John Bolton, began to pick up momentum during the Biden administration, when U.S. intelligence officials collected information that appeared to show that he had mishandled classified information, according to people familiar with the inquiry.

The United States gathered data from an adversarial country’s spy service, including emails with sensitive information that Bolton, while still working in the first Trump administration, appeared to have sent to people close to him on an unclassified system, the people said, speaking on the condition of anonymity to discuss a sensitive case that remains open.

The investigation of Bolton, who has become an ardent critic of the president, burst back into pub

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