In a statement Thursday, the Secret Service said it was “aware of this matter and will conduct the appropriate investigation.”

Hope Hicks, a Trump campaign spokeswoman, said in an email about the posts by Mr. Senecal, “We totally and completely disavow the horrible statements made by him.”

Mr. Senecal did not immediately respond to a telephone call and Facebook message seeking comment.

The posts were revealed as Mr. Trump, the presumptive Republican nominee for president, has come under scrutiny for his associations with, and support from figures such as the former Ku Klux Klan leader David Duke. On Tuesday, Mr. Trump’s campaign aides said a technical glitch had included William Johnson, a self-proclaimed white nationalist, on a list of California delegates they submitted to the Secretary of State’s office.

Mr. Senecal had worked almost 30 years for Mr. Trump, and was the subject of a profile in The New York Times in March. He spoke of knowing Mr. Trump’s sleeping patterns, his culinary preferences and just the right ways to cheer him up when he was in a sour mood, once hiring a bugler to play “Hail to the Chief.”

When he tried to retire in 2009, Mr. Trump deemed him irreplaceable and kept him on as an unofficial historian at the Mar-a-Lago estate in Florida, the article said. But in her statement, Ms. Hicks said, “Tony Senecal has not worked at Mar-a-Lago for years.”