Abbe Lowell, one of the younger Biden's lawyers, cited past examples when Trump's language allegedly inspired violence, notably the Jan. 6 attacks on the Capitol, the October attack on Speaker Nancy Pelosi's husband Paul Pelosi, and the man arrested near the Obamas' Washington, D.C., residence last month with an arsenal of firearms.

"This is not a false alarm," Lowell wrote. "We are just one such social media message away from another incident, and you should make clear to Mr. Trump -- if you have not done so already -- that Mr. Trump's words have caused harm in the past and threaten to do so again if he does not stop."

Lowell also defensed his client against suggestions made by Trump and others that cocaine found at the White House earlier this month might have belonged to Hunter Biden, who is a recovering drug addict.

The Secret Service closed its investigation into the matter on Thursday without identifying a suspect.