The post was only visible to his Facebook friends, but Senecal confirmed its authenticity to Mother Jones, saying, "I wrote that. I believe that."
"The U.S. Secret Service is aware of this matter and will conduct the appropriate investigation," agency spokesman Robert Hoback said in an email Thursday.
The Trump campaign denounced Senecal's messages and distanced itself from the former butler at Trump's estate, the Mar-a-Lago in Palm Beach, Fla.
"Tony Senecal has not worked at Mar-a-Lago for years, but nevertheless we totally and completely disavow the horrible statements made by him regarding the President," Trump campaign spokeswoman Hope Hicks said via email.
A 1,600-word New York Times profile published in March reports Senecal as still having a presence at Mar-a-Lago after his 2009 retirement from official butler duties.
In "A King in His Castle: How Donald Trump Lives, From His Longtime Butler," New York Times reporter Jason Horowitz describes Senecal's relationship to Trump, writing "few people here can anticipate Mr. Trump’s demands and desires better than Mr. Senecal, 74, who has worked at the property for nearly 60 years, and for Mr. Trump for nearly 30 of them."
When asked whether Senecal is still employed by Trump and receives a paycheck for working at the estate, Hicks responded: "He is not employed by Mr. Trump, and hasn’t been since June of 2009."
She added: "We strongly condemn these horrible comments from Mr. Senecal."
Wednesday's Facebook post isn't the first time Senecal has called for Obama to be killed, Mother Jones reported. He also has made threatening remarks about first lady Michelle Obama. From Mother Jones:
Senecal regularly posts screeds on his Facebook page from a far-right perspective in which he decries Obama and his wife—along with Hillary Clinton, other Democrats, and Republican leaders. He often refers to Obama as "zero," and several times he has called for the president's execution. He confirms that he has written all the posts on the page that have appeared under his name. "It's all me," he says.
Janell Ross contributed.