Former vice presidents typically get federal government protection for six months after leaving office, while ex-presidents do so for life.
But another person familiar with the matter says then-president Joe Biden quietly signed a directive that had extended protection for Harris beyond the traditional six months.
Sources insisted on anonymity on Friday to discuss a matter not made public.
The move to drop Harris' Secret Service protection comes as the former vice president is about to embark on a book tour for her memoir, titled 107 Days.
Harris became the Democratic presidential nominee last summer after a chaotic series of events that led to Biden dropping out of the contest.
The book tour has 15 stops, including visits abroad to London and Toronto.
The book, which refers to the historically short length of her presidential campaign, will be released September 23, and the tour begins the following day.
It is not unusual for Secret Service protection to continue well beyond the statutory six-month window, particularly when former officials face credible and ongoing threats.
But Trump's moves have stood out both for timing and for targets.
A senior Trump administration official said an executive memorandum was issued on Thursday to the Department of Homeland Security, ending Harris' security detail and security services.
They would have ended in July 2026, but now they will be terminated on Monday.
During Trump's second presidency, he repeatedly cut off security for figures who had fallen from favour, including his one-time national security adviser John Bolton and members of Biden's family, including the former president's adult children.
The decision to strip Harris of protection is certain to raise alarms among security experts who view continuity of protection as essential in a polarised climate.
Last year was a particularly politically charged environment, with Trump facing two assassination attempts, and the Secret Service played a crucial role in protecting the now-president.
Harris is a former California attorney general and San Francisco district attorney.
While she lost to Trump last November, Harris is seen as a potential candidate for 2028, and she has already announced she will not run for California governor in 2026.
Harris is also a former senator, California attorney general and San Francisco district attorney.
The news of the security revocation was first reported by CNN.
7°C