Charleston police have received nine reports of anonymous letters in recent weeks that contain general threats of racially and religiously motivated violence, with one note mentioning Dylann Roof by name.
The police department and the FBI have opened a joint investigation.
"We take all incidents of this type seriously and investigate them thoroughly," Charles Francis, spokesman for the Charleston Police Department, said in an email.
Several managers at hotels that received the letters told police the notes appeared to have been sent from out of the country. Some envelopes were postmarked in England, and a few had Canadian stamps. They did not include return addresses.
The majority of the letters were addressed to hotels in downtown Charleston, and one was sent to James Island County Park. Staff at Emanuel AME Church, where Roof, a self-avowed white supremacist, is accused of gunning down nine black parishioners in June 2015, also reported receiving two harassing letters in October.
A handwritten note sent to Renaissance Charleston Historic District Hotel last week said "Every one of the 9 lives sacrificed by his greatness, Mr Dylann S Roof, is one less I will need to sacrifice," according to incident reports. A pistol was drawn below the statement.
Other letters contained threatening statements toward Jewish people, blacks, Hispanics and Muslims, with some referencing violence at a synagogue and a mosque.
None of the threats contained in police incident reports detailed specific plans or targeted specific individuals.
The letters were received ahead of jury selection this week in Roof's federal death penalty trial on 33 charges, including violations of hate crime laws and religious freedoms.