President Trump has long had, at best, a cavalier attitude about the handling of classified material.

In his first term, he took a photograph of a satellite image of an Iranian launch site that had been included in his daily intelligence brief and posted it on social media.

During a meeting in the Oval Office, Mr. Trump shared a tip with the Russian foreign minister from Israeli intelligence about a terror plot — then asked intelligence officials to go on television exonerating him.

And when Mr. Trump left office, he stacked classified documents — and other records the Justice Department said had been improperly taken from the White House — in a bathroom at his Mar-a-Lago resort in Florida.

Now, Mr. Trump’s attitudes and actions are coming under renewed scrutiny as the national security adviser, defense secretary and multiple cabinet members face questions about their use of Signal, a commercial messaging app, to discuss details of impending military strikes many experts say were classified.

The conversation was described in an article Monday by The Atlantic’s editor in chief, who was mistakenly added to the group chat.