Senior American officials were worried. Since the early months of the Trump administration, Jared Kushner, the president’s son-in-law and Middle East adviser, had been having private, informal conversations with Prince Mohammed bin Salman, the favorite son of Saudi Arabia’s king.

Given Mr. Kushner’s political inexperience, the private exchanges could make him susceptible to Saudi manipulation, said three former senior American officials. In an effort to tighten practices at the White House, a new chief of staff tried to reimpose longstanding procedures stipulating that National Security Council staff members should participate in all calls with foreign leaders.

But even with the restrictions in place, Mr. Kushner, 37, and Prince Mohammed, 33, kept chatting, according to three former White House officials and two others briefed by the Saudi royal court. In fact, they said, the two men were on a first-name basis, calling each other Jared and Mohammed in text messages and phone calls.

The exchanges continued even after the Oct. 2 killing of Jamal Khashoggi, the Saudi journalist who was ambushed and dismembered by Saudi agents, according to two former senior American officials and the two people briefed by the Saudis.

As the killing set off a firestorm around the world and American intelligence agencies concluded that it was ordered by Prince Mohammed, Mr. Kushner became the prince’s most important defender inside the White House, people familiar with its internal deliberations say.

Mr. Kushner’s support for Prince Mohammed in the moment of crisis is a striking demonstration of a singular bond that has helped draw President Trump into an embrace of Saudi Arabia as one of his most important international allies.

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